hello,
thanks for fast response. it had worked only in browser windowed mode.
when setting to full screen, youtube automatically dropped down from 1440 → 1080 → 720p…
so i did an full uninstall of both gfx drivers, local cpu AMD and RTX3070 Nivia Drivers with newest Display Driver Uninstaller. cleaned and rebooted both uninstalls twice. Then installed only the newest Nividia Driver, didnt changed anything in the new driver with Nvidia Control Panel or the Profile Inspector, just left it as it was
success 
Thanks alot, cheers ^^