i wrote it myself a long time ago. It doesn’t use a rigidbody. the physics are raycast based (capsule casts really) and fires a bunch of those every tick to do the movement. the physics needs a rewrite incidentally it has a lot of degenerate cases where you just fall through the map
The movement code is pretty similar to https://github.com/id-Software/Quake-III-Arena/blob/master/code/game/bg_pmove.c the physics are pretty similar to what I would guess this asset does https://assetstore.unity.com/packages/tools/physics/kinematic-character-controller-99131