Two questions about NFTs, please help me understand?

I've been reading about NFTs and I think I got a basic understanding. But there's two things I can't wrap my head around.

1. What exactly ties an NFT to what it's for? Like, if it's for a digital image, is the image file included in the NFT? Or does the NFT only have a url pointing to the file hosted somewhere? What if in the future, that url recorded in the NFT hosts something other than the original file? And in case the NFT is for a physical item, how does that work at all?
2. How is the issue of ownership resolved during the creation of an NFT? Can I just create an NFT of a random image I found off the internet? Can I create an NFT for "google.com", or "the physical copy of the Mona Lisa"? I guess not, but what exactly prevents me from doing that?

Thank you!