I still have the video I've sent to them at some point, it describes it in all detail, if you can bear my accent..
I’ve had laptops before where the video ports would only connect to the dGPU, and the internal screen used Optimus (display output from the iGPU with graphics acceleration from the dGPU on demand). Lots of dual GPU laptops are MUXless like that in fact.
Yeah, I've had some of those. Actually owned one of the first generation optimus laptops and it was horrible, most of the time it did not pick up the heavy load and stayed on iGPU even when playing games. Seems to be much improved a lot in win10-11, but I still prefer the kill-switch.
This one kind of works like that too, though. The MUX only controls which GPU the main panel is connected to (and with it, the framebuffer). The modes basically are:
"Eco" where only iGPU is enabled
"Hybrid" where iGPU is main and maintains framebuffer while offloading work to dGPU when needed just as you've described
"Ultimate" with Nvidia as main, which apparently gives much better framerate and latency because it does not require overhead of workload offloading and framebuffer shuffling, but the dGPU is by far the most power hungry device at 150W TDP which drains the battery in mere minutes, even on idle
I have had issues with dual GPU systems like that on Linux
I feel you. My previous setup was a desktop with both AMD and Nvidia cards, which I juggled between the host and VM. It was pain, mostly because Nvidia did not want to play nicely. Also because most utilities assumed I had Intel APU — I didn't, but it was fair assumption at a time. Nowadays, it seems like everything's sorted out, even VFIO was a breeze to set up (though what for, most games now play on linux nowadays thanks to steamdeck)