Users have the option of setting up two-factor authentication. The setting for this is set up in “My account” by the user him/herself. This requires an OAUTH authentication app (e.g. Google Authenticator).
For administrators, the function “Login as …” is available in the individual users section under Administration > User Accounts & Groups. This can be called up via the detailed view of the user (top right side).
If this function is used, the view appears with a red frame. Furthermore, it is shown at the top right that you are currently in this mode. There you can also switch back to your own user.