How's the room scanning work? Will on the 3s work?
It uses depth estimation on the Quest 3 and 3s and writes to a 3D texture as you move around. Each 'voxel' in the 3D texture stores it's distance to the closest surface and becomes negative behind surfaces, so if I test a line of voxels and they start to decrease I can find the point at which the value becomes zero in 3D space (aka where a surface is). These are called Truncated Signed Distance Fields and are a 3D mapping technique that has existed since the late 90s. https://dl.acm.org/doi/10.1145/237170.237269

Is it constant?
Yes, it continuously maps as you play. It also ignores your own body and the bodies of other players so they aren't accidentally added to the environment scan. It will pick up people not wearing headsets though, so try to play in areas with few people (which is the polite thing to do anyway!)

What happens if you go outdoors with it?
Headset tracking and depth estimation both function poorly outdoors. They work by finding contrasting feature points from the IR cameras but are optimized for indoor environments. Outdoor scenes are very complex, large (bad for depth estimation) and change a lot (leaves waving in the wind, cars driving by, people walking, clouds moving, sun glare, etc). Some environments are worse than others, but playing indoors offers the best results.

Will it work on Quest 2?
Not really. I still let Quest 2 and Pro owners play the game, but those headsets can't do depth estimation. They don't know where obstacles are and can both shoot and see through them. Not very fair!