Earlier quoted context omitted.
At least for Gitlab, you can write a .netrc in the user‘s home dir like so: machine (e.g. gitlab.com) login password For the access token, you can also leverage Gitlab‘s CI Job Token. What we do is an „echo $(netrc contents with $CI_JOB_TOKEN) > ~/.netrc“ in the pre-script in CI. ( https://stackoverflow.com/a/61257782 and https://docs.gitlab.com/ee/security/token_overview.html )
If you do take this approach, writing your access token in plaintext into a well-known file in your home directory seems like a strictly worse idea than the (also more broadly-compatible) Git insteadof rule to pull via SSH and your local agent.
Yes, though it should be noted that the access token is only valid for the duration of the Job.