Air units typically have a low metal cost, and a very high energy cost. Most T2 aircraft require much more metal than T1.
A special 'armor class' reduces damage dealt to aircraft by most weapons that aren't dedicated AA.
Because flanking angles are represented in three (3) dimensions, being hit by aircraft causes this angle to point skyward and follow it, potentially enabling side flanks by ground forces.
Units have an "Air LoS" (Line of Sight) stat in addition to their normal sight range stat. Air LoS is typically higher, allowing the unit to see aircraft at a greater distance than they would spot a ground unit—i.e. a missile turret will see incoming bombers long before it would see tanks.
While both factions have aircraft to fill the basic roles (see "Types" below), Cortex's tend to have somewhat better stats than Armada's equivalent—their bombers most notably hit harder, especially in T2. Armada has a much more diverse air force in T2 between EMP bombers, surgical strike gunships and tactical nuke bombers, while Cortex focuses more on having better mainline units, and the Dragon superheavy gunship.
Unlike most ground units & ships, air units never leave a wreck when destroyed, making it impossible to reclaim any metal from killed aircraft. Some, like gunships, may plummet to the ground and explode, potentially damaging nearby units.
- The exception to the no-reclaim-for-aircraft rule is if they are reclaimed while still alive. This can happen even mid-flight. Otherwise idle Construction Turrets will automatically reclaim enemy aircraft in reach if possible. This tends to happen only incidentally, however, and is difficult to plan around.

Types

Scouts fly quickly to explore the map. This includes radar/sonar planes, which, given their position in the air, have an edge over ground radar because they're less likely to be blocked by terrain.
Fighters engage other air units with air-to-air homing missiles. T2 Stealth Fighters are invisible to radar on top of being stronger overall, adding ambush capability.
Bombers drop bombs on surface targets. Most drop bombs in a line, though a few drop in a spread or drop only single bombs. Torpedo bombers instead drop homing torpedoes to attack sea targets.
- Bomber projectiles are subject to gravity, which is noticeable on low-gravity maps like Comet Catcher, where bombers may overshoot their target due to drift.
Gunships hover as well as fly, and are generally not as fast as planes. They engage surface targets, though torpedo gunships can also attack subs. Gunships tend to have much more HP, but lack air-to-air weapons.
Transports ferry land units across the map. The heavier the unit carried, the slower the transport moves. The max weight of the units they can carry depends on the type of transport. Weight is a separate, hidden unit stat different from metal cost. The difference is most notable with superheavy tanks like Tzar or bots like Mammoth, which require a heavy transport. Transports are unarmed, with the exception of the Abductor.
Constructors can fly & hover over any terrain like Gunships, but are relatively slow compared to other aircraft, and their buildpower is extremely low. It is often more efficient to use them to start buildings, and then let airlifted Construction Turrets do the work. They are also highly vulnerable to damage. However, they're also the fastest out of all constructors, and perfect for quickly reclaiming wrecks in out-of-the-way spots.

Movement

Planes fly in patterns when idle, and land on the ground if enabled to with the Fly/Land toggle unique to air units.
Seaplanes fly like planes, except they can also land underwater, making them targetable by torpedoes and similar weapons.
Gunships hover in place when idle. When moving they can turn and fly other directions without turning in long, sweeping arcs like planes. They can also land just as planes/seaplanes can.

Air Transports

T1 light & heavy, and T2 heavy air transports are available to all factions.
Light transports can carry any units with the "Transportable: Light" feature. They can also airlift Construction Turrets. Notably, light transports cannot carry Commanders.
Heavy transports can carry units with the "Transportable: Heavy" feature, including but not limited to Commanders. Many T3 units are still too massive to be carried at all. Heavy transports can also airlift certain T1 static defenses, including LLTs & HLLTs/Beamers, missile turrets, and radar towers.
While carried, units cannot move or attack in any way, and all special abilities (i.e. radar, cloaking) are turned off.
Armada's Abductor confers its radar stealth property to any unit it carries, thus allowing stealthy insertions near enemy territory—though remember that most units still have a longer Air LoS stat to spot aircraft with!
If a transport dies, any unit it is carrying also dies—except the Commando, which drops to the ground and can then move and fight as normal.
A unit must typically hold still for a transport to pick them up. Units will not automatically hold still if a transport is trying to Load them, so the owner must order them to stop.
Transports can pick up and carry enemy units, too!

Air-focused units, buildings & defenses

AA (Anti-Air) comprises a wide variety of static buildings from basic missile turrets, to mid-range flak, to long-range SAMs that can take multiple planes out in one shot. Dedicated anti-air are most common among mobile units, but some, particularly Armada T2 vehicles, possess light anti-air missiles in addition to their ground attack.

Missiles tend to have high range, and are good at bringing down individual planes. However, few missiles have AoE. LRAA (Long-Range Anti-Air) missile turrets must stockpile a limited arsenal of extreme-range, high-AoE missiles, which they can then fire in quick succession. LRAA is good for blunting the force of an incoming push, but their shots can be baited out by individual planes if not set to Hold/Return Fire.
Flak tends to have shorter range compared to missiles, but it fires quickly and with high AoE, shredding groups of air units. They're especially good vs. gunships.
Other units that have no anti-air weapon can still sometimes fight back vs. air. The Commander's laser can easily destroy bombers, for instance. Laser-armed units in general tend to have more luck than slower projectiles, although they are rarely more effective than dedicated AA, and aircrafts' special 'armor class' often reduces the damage dealt.

Air repair pads exist to rapidly repair air units with their 1,000 Buildpower. Air units can be set to automatically land on the nearest pad once they reach a certain health percentage for repairs, after which they return to the fight. This is usually only useful for gunships, because most planes have very low HP, and thus their missions are typically do-or-die strikes into enemy territory.

Tactics

Large team games (8v8, etc.) typically have at least 1 dedicated air player. This gives early access to scout planes and air transports, which are arguably more important than bombers due to their unparalleled ability to gather intel and reposition units. It's common to see players ask for a heavy air transport to carry their Commander to the front line. The commonly-accepted meta in many games is similar to trading constructors one couldn't get in the early game—to build a few Wind Generators, and then gift them to the air player in exchange for a scout plane or transport.

Mid- to late-game bomber strikes typically consist of a screening force of Fighters sent ahead to soak up hits and combat any enemy Fighters, while escorting Bombers so they can deliver their payload. This is typically countered by having large squadrons of Fighters on long patrol routes to form an "air-wall" against these attacks, and by having enough flak to decimate packed aircraft.

Switching to air is an often risky, but potentially highly rewarding move, especially in games with less players. Building a small fleet of bombers in secret and using them to decimate your enemies' economy, bombing masses of wind generators and destroying Construction Turrets, can drastically hurt their strategy.

Gunships are good for "catching leaks", i.e. for destroying fast units that bypass the front line and pierce through to one's base or economic structures.

Cortex's Shuriken EMP drone is so unique that entire strategies can revolve around it. They can stun enemy units, rendering them vulnerable to follow-up attacks, and their AI auto-retargets units that haven't been stunned yet if they have no dedicated Attack order. It is so potent that several T2 Armada vehicles (e.g. Jaguar) incorporate AA just to deal with them. However, Shuriken die in a single hit from even light AA missiles.

Keeping a reserve force of T2 Stealth Fighters while you still have a visible group of T1 Fighters can be useful vs. human opponents, to fool them into thinking that is your only screening force against bombers. Your stealth fighters can then attack from a flanking angle when enemy air approaches, killing them with near impunity because air attacks typically do not have the luxury of turning to face a threat from the side, as they always rush headlong to escort bombers through to attack bases.

Using light air transports to carry Construction Turrets to critical battlefield locations can be highly effective. This can set up repair stations, rapidly reclaim wreckage fields, or quickly set up buildpower to bolster a factory or establish defenses. To avoid having to use all their own resources to assist-build, the air player can gift these turrets to the player they're dropping them for.