You can build your own lower receiver, too. You can buy it as a finished receiver and parts kit, then put it together yourself; you go through an FFL dealer when you do that, too. But some people are handy and enjoy a challenge, so they go out and build their own lowers.
Some people *really* build their own lowers.
Like this guy, who whittled one. Well, okay, he milled it, but still, he built a functioning AR-15 receiver out of a block of wood.
It only lasted three shots, but hey, he built it.
(It's quite legal to build your own firearms, provided you don't build full-autos. If you do that, you have to get the permission of the ATF, and they'll only give it if you're making it for a government agency. When I have a house and can actually set up a shop, I might try my hand at gun-building.)
