Hi ,
Twinmotion uses both your system and GPU RAM, generally for different reasons.
Your system RAM comes into play mainly for loading objects and assets into the scene. The more assets you have, the more system RAM will be utilized. RAM usage will usually peak during the file load and save process.
Your GPU VRAM will be used for primarily for rendering tasks, such as rendering your image to the viewport or exporting renders.
DirectX12 does use more system resources, both RAM and VRAM, than DirectX11, so if you have models that don't load due to running out of memory, switching to DirectX11 mode should allow you to open your files again. Once open, you can remove the most resource intensive elements of your model and then try switching back to DX12. Disabling Path Tracer in your saved media, removing HDRIs, and removing lighting or vegetations assets can make the largest impact. Making sure you use instances of duplicate assets, rather than copies or unique elements, where possible can also help reduce resource usage.
Best,
Raghib