Mageia forum

I have a small PC (Intel NUC) and workstation with Mageia installed. There are several users (A, B, C, ...) on the small PC and user C also exists on the workstation. How can I ensure that user C, when he logs on to the small PC, is automatically logged on to the workstation remotely is and only sees the GUI of the workstation? The user should have the feeling that he is logging directly into the workplace. Both systems are connected via a point-to-point network.

That could be a case for XDMCP - although that is pretty insecure as it is unencrypted, and should only be used on private networks.
See also https://www.codingdomain.com/linux/remote/x11/ for an overview. You also have the option to use stuff like x2go or VNC or xrdp or such.

Although it might help if you would provide some more context information around your use case, what you actually want to achieve and why.

The workstation is in a locked room and the small PC has a direct network connection with this workstation. The "net" consists only of these two systems. However, user C should be able to work on the workstation as if he were logging in directly there.

Well, the last part might be a small problem. X forwarding usually has some issues with e.g. stuff that regular users often use, like drag&drop or multi-monitor setups and such. You might need to try out what works for you, if XDMCP is enough.