Why can you build apps on ethereum but not bitcoin?

Technically, why?

I’m not very knowledgeable yet on the technical differences between bitcoin and “utility” protocols like ethereum besides simple stuff like block size and proof of work/stake.

What does it even mean to “build” an app on a blockchain? A blockchain itself is essentially just an encrypted ledger. Where are these apps being built?

Are NFTs just new, unique tokens that you can track in the transaction list of an ethereum block like any ordinary ether?

I think a good explanation around this broad concept would help a lot of people like me. Are there any resources you’d personally recommend?