How to set up file copying between servers using keys?

Question

SSH

There are 2 Debian servers that you can ssh into using the same private key. How to set up copying a file from server 1 to server 2 via scp or rsync using the installed keys without entering a password?

Answer 1

server 2 only needs to know the public key of server 1, so just copy it using ssh-copy-id or manually write id_type.pub into authorized_keys by condition, it turns out that's done.
Shl but server 1 must have this private key. Although it is more correct not to copy the private key to server 1, but to generate a new one on it (or use an existing one) and transfer its public part to server 2