Setting up SSH keys for all my computers

Hi everyone, every time I setup a Pi project server I have a hard time since I can only access that device from the computer where I created the image since I add the SSH key during setup, but I would like to be able to ssh from my android and my other laptop in case I'm out on the road and away from home.

How can I add the other keys for the other devices to a single Pi without using the user name and password as this is also given me problems because ssh wants to try using keys and if it does not find them it just fails instead of asking me to enter the password (also, because it's not really safe).