Earlier quoted context omitted.
How does key exchange work, and does the server have copies of the keys?
You can find more info on key exchange here - https://github.com/RNCryptor/RNCryptor . The server does not have copies.
No, you really can’t. That’s a library for symmetric cryptography. The key distribution/exchange problem cannot be solved with symmetric cryptography alone - unless you are acting as a trusted party and conveying the symmetric key - in which case you are not implementing end-to-end cryptography.