cdwifi

It's a simple question: how do they enforce the law or do they not?

Fair enough, though that would suggest it won't be enforced as it cannot be enforced. As long as it is about a software repository (aka "app store") under US jurisdiction they can still force them to take the application offline.

What do they do with foreign "app stores"? What do they do with websites?

Apart from the the possibility of network filters, the whole thing also shows the fundamental issue with centralised and monopolised software respositories, as promoted by Google and Apple.

If you can download and install an application from any website (as was the standard approach in 90s and 2000s), such censorship (be it corporate of governmental) would not be possible.

That is true, but unless they do not enforce the law, they will still need something to make sure such "relatively large sites" cannot be accessed. This is where the OP's comments come in.