Why does that appear on the screen?

I have a Huawei P30 Smart.

Actually, when I switch between two apps, everything runs smoothly, but sometimes it is just that a small part of the previous app can still be seen very faintly (for a maximum of one second). Most of the time I notice that in the top right. An example would be the round profile picture at the top right here in the Girlfriend app, which then briefly shimmers through in Whatsapp.

I had that on my old cell phone and put it on the outdated technology back then, but now I can't believe it's normal.

Can someone explain to me why this is the case and best of all how it can be prevented?

Can have many reasons.

Perhaps the cell phone does not fully follow and still shows an old frame in between.
Maybe your display is a little broken and you have dead pixels. Did that on my Huawei, even if something is displayed for too long, it takes again until it goes away. Because the pixel was used too much.

That should be on the display. With many displays, it is normal for eye-catching colors to burn into the display for several minutes.