In addition to ssh-copy-id that is already mentioned is ssh-import-id which copies a key from launchpad.net or github.com.
It is available in ubuntu and maybe elsewhere by default.
It is roughly equivalent to curl -s https://api.github.com/users/$USER/keys | jq -r ..key >> ~/.ssh/authorized_keys assuming your username is the same on github as it is on the linux system.
curl -s https://api.github.com/users/$USER/keys | jq -r ..key >> ~/.ssh/authorized_keys
Does that mean that Github ir Launchpad gets access to the files on your computer?
No. The ssh-import-id runs on your computer and calls remote API and writes the file on your computer.
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