You need the decryption keys, and yuzu only gives you instructions on how to extract them from your own switch (and is careful to stress that point). But nothing stops you from searching for the keys on the web and getting them from there.

You don't. I'm not aware of a way an emulator could confirm you bought the files you give it to run.

Oh, I didn't know you had to own a Switch. I thought you just had to own the game?

Yeah, like the guy above says. They don’t actually circumvent the DRM. They just get the switch’s key. I think there are only as many keys as there are switches, so while you could probably buy it on the gray market, with just yuzu and a site that hosts roms, you can’t do any pirating