Windows 10 - Do users have access to all devices?

I borrowed a laptop. At the top right it still says "User XY is logged in" (or something like that). A separate account was opened for me, with which I register myself. However, user XY remains visible and exists as a logged-in user. Can this user see mine from his device, or can he access my documents himself and see when I'm working on the laptop? Or is this "user" only important because of the license or if he would log on to my laptop himself? Or does he have no insight at all?

You can't say it like that. This is determined in the settings and there are countless graduated options for what and who is "visible" where. In combination, there are certainly tens of thousands of variants of the setting.

Thank you, Mauritan, you have helped me a lot and confirmed my assumption that this is basically possible.