How can I use my 2 monitors completely independently of each other?

I'm using a 4k and a FullHD monitor together since today. Now I would like to know how I can use the two independently.

Because if I When I start a game on my main monitor, all the windows on the other monitor close and the contents that were previously on the main monitor are displayed. Provided that it was not in full-screen mode (for example, YouTube or Netflix).

Of course, the display setting is already set to extended display.

I hope someone finds some time to help me with the problem.

Do you have the program "Nvidia Control Panel"?

Jap, I have

But you have already changed with the key combination Windows + P on "expand" or?

Do you start the games in full screen mode?

Do I have

Yes

Have you ever tried to mark a monitor as primary?

The 4K monitor is the primary screen, it should be like that.

Try to start in window mode

I have and it works so far, but there were some problems in windowed mode in every game I tested.

Actually synonymous give a frameless window mode, vlt works better with it