The lower receiver of an AR15 is legally considered the firearm. You can buy all other parts straight up, but you have to go through federal background checks on that one. Even with private sales, at least the first buyer would have to have gone through the process.
On its own, it's just a chunk of plastic or metal. It's not pressure bearing and isn't even all that mechanically stressed in typical use. Therefore, you can print that one part off, buy all other parts, bypass all checks, and have a completely unregistered AR15. It's not especially difficult to do, though it does involve a few specialized tools.
In the UK, regulation tends to be around pressure bearing parts, and this is a lot more sensible.