cat ~/.ssh/authorized_keys command shows you the authorized_keys file of the currently logged in user. When logged in as root, or using
sudo, this will give you the authorized_keys file of the root user.
The authorized_keys file, at least on Ubuntu, is usually owned by the user. So the currently logged in user (root or not) can see it.
The .ssh directory is in the user's home directory, and usually owned by them with read, write and execute privileges; so normally a user should be able to indeed add their own authorized_keys file.
To see all authorized keys, you could just create a script that iterates over all home directories and /root, and prints the .ssh/authorized_keys file. Obviously this script will require sudo privileges.
As a side note, on Ubuntu the root account is usually disabled, because it is a favorite target of attackers. It may not contain an authorized_keys file for this reason.