XPS 9575 2-in-1 Radeon RX Vega GPU not being used

Depends on where  you're looking.  You need to check inside the AMD software  to verify.  If you're looking under Windows itself, note that all video data (including that passed through the AMD GPU) is sent through the Intel GPU on its way to the screen, so Windows will always see only the Intel GPU as active.

This is the way the system is designed to operate.