Password

Updates won't change your user and settings, so user and password remain the same.

If you make a clean install then you'll have to set it all up again but you can use the same user and password.

If you recover from a Time Machine or if you use Migration Assistant, then your old settings will be brought back and you'll have same user and password as before.

<<Or does my password only ever change if I personally change it?>>

^-- THAT.

By design any Install, re-Install in place, or upgrade of MacOS bulk-erases only system directories. By design, all of your Files, added Applications, Preferences, and passwords are intended to remain undisturbed.

That said, re-installing MacOS writes over 350,000 files. Occasional problems can and do occur. You should be certain you trust your recent disk-based backup before making a major upgrade.