Take a look at his sample code: https://developer.apple.com/documentation/realitykit/rendering-stereoscopic-video-with-realitykit

If you're looking for a programmatic way of capturing spatial videos, check out https://developer.apple.com/videos/play/wwdc2024/10166/?time=6m39s

VNGenerateForegroundInstanceMaskRequest is what you want. https://developer.apple.com/videos/play/wwdc2023/10176/ explains how to do it.

If the foreground object is a person, you can use this sample code. https://developer.apple.com/documentation/vision/original_objective-c_and_swift_api/applying_matte_effects_to_people_in_images_and_video

This sample code may help.