On Towards the Stars: Planes and Anti-Air
[This post is part of a larger strategy guide for Civilization 6]
A lot of people really sleep on planes — I think because the AI never builds them — which is unfortunate because they're actually really powerful. They can never be attacked by existing land units, they have a massive range, and they do a fair bit of damage. Tldr: build planes, they're good.
Planes are interesting because you can't upgrade into them. The traditional strategy for maintaining a large standing army is to build a bunch of cheap ancient era units — warriors, slingers, horsemen — and upgrade them through the tech tree. You can't do that at all with planes. In other words, a defender with a few planes going against a warmonger without planes has a fighting chance (or, put another way, any warmonger also needs to have some production to build new units)1.
In the late game, fighters, bombers, anti-aircraft, and nukes have a bit of a rock paper scissors thing going on.
Bombers will instantly pillage districts and can carry nukes (though they will not be able to launch the nuke if a fighter or anti air gun brings its health down to 50 or less);
Fighters can do a lot of damage to bombers and have a chance of taking down nukes coming from bombers if the fighter is deployed in a one tile adjacent area;
Anti-air defends all tiles in a one tile radius from any kind of plane (and mobile SAMs also can protect against nukes) but that's all they do. Besides the actual anti-air class of units, you can get anti-air defense from battleships (and up) and GDRs (see below). The actual way anti-air gets calculated is pretty complex and almost never really matters; still, if you're interested, check out the civ wiki.
Generally, you want to treat your bombers as siege, fighters as ranged, and anti-air as counter spies protecting key assets. I tend to use bombers to go after aerodromes, factories, and commercial hubs to try and cripple production during wartime; and fighters to help on the ground or defend against enemy bombers.
It's worth mentioning that by the time you're slinging planes, there's a nonzero chance that there's a GDR or two in the game. GDRs also stop nukes, and are much more susceptible to fighter class planes than bombers. But they also carry anti air guns, and do a fair bit of damage themselves. If you're going against a GDR, make sure you have a lot of jet planes lying around.
Note that, like all units, planes are less powerful the faster the game is. Playing on online speed will mean your planes are less valuable. It also means a war-monger might be able to win before planes really slow them down.