I am currently using FileZilla to see my files and when I log in to each user separately, it shows authorized_keys is already created and owned by account a and therefore is only used by account a (so I can’t just create a new file on a different account), what I want to do is change that so that both root and account a use the same key, but not account b. I will also want to install a key just for account b later.

After I typed out this response, I thought of using ssh-copy-id, but that did not work.
I also thought about changing the sshd_config file so that it specifies key files to use with specific accounts, is that possible?

I was just looking around in my files and realized that each user actually does have their own folder for ssh keys, but for some reason, I had previously changed the owner of the root key to account a, I have changed the owner back to root and the same key is already in account a’s ssh folder, so the same key is now being used for root and a. Thank you for your assistance.