You should do one private key per host. If you want access to something with a key from you phone and computer you should have one key on each. To use ssh from somewhere, from that place to your computer, gitlab, whatever, it needs the private key in the correct place. If it is a shell (like termux) it should be e.g. yoir home folder then .ssh/id_rsa. If its an app, I guess ot tells you(, but I would not give my private key to a random app). To be able to get to somewhere, like from your phone to your computer, your computer needs the public key. The cannonical place to put it is as a single line in a file 'authorized_keys' in the .ssh folder in your home folder.