Thank you everyone for explaining your view of how it works on Linux, it seems to be very chaotic when it comes to this topic, everyone has their own understanding and some contradict each other. But it was great to get some feedback and food for thought which let me to try other things which I didn't think of trying before.

Now I finally have some things working on the iGPU because I plugged the monitor into the motherboard instead of the dGPU. It seems that the offloading works well still because things like ollama and video editing still are done on the powerfull dGPU and then somehow the results synchronized to the motherboard output and on the screen.

When it comes to the iGPU I see that many applications run on both at the same time like for example Firefox or Chrome when I look at nvtop. Especially when I watch videos in the browser then the iGPU is mostly active. Also when I use GNOME Shell features like seeing the overview by pressing the Super key or switching between virtual desktops.

With help of switcheroo-control I can now force a program like DarkTable to work specifically on the dGPU.

So while I still don't have full controll over it, I feel I'm utilizing both graphics cards now much better and that was what I was looking for before I wrote this question, so thanks a lot everyone!