Looking for high-level answers on some of the mechanics behind consensus concepts

1) Can someone explain to me what peer-to-peer actually means, both apart from and in the context of block chains? P2P wasn’t invented with Bitcoin was it?
2) What is a merkle tree and how does it work in the context of Bitcoin? What does it do? Why is this concept important?
3) In digital signatures, how is it possible for me to sign a transaction with my private key and not have it publicly shown to everyone?