Currently - I don’t think so, but there were plans to make some of them public
Yes (most of the time). The interface definition is inside the canister metadata. Look for a canister at dashboard.internetcomputer.org and you will see its interface.
There is also this library that helps with IF discovery - icblast https://github.com/infu/icblast
Tool - Blast https://jglts-daaaa-aaaai-qnpma-cai.ic0.app/
VSCode extension https://forum.dfinity.org/t/internet-base-vs-code-extension/20080/11