Basically it works by setting the OpenXR environment blend mode to be alpha blended, and setting the output alpha on the output framebuffer to 0 where you want the passthrough to be visible.
I haven’t made any tutorials on this myself yet, but Varjo (who’s OpenXR runtime supports this method natively) have some tutorials on how to set it up in Unreal. Note that the IsMixedRealitySupported and IsMixedRealityEnabled nodes require Varjos own Unreal plugin.
Basic setup:
Masking out specific objects: