Windows Mixed Reality Controllers Working

I was under the impression that UE4 used SteamVR to view VR projects. Yesterday my SteamVR stopped working after loading an environment from their store and I was getting a 301 error. However when I opened up my UE4 project and played it in VR it still displayed on my Samsung Odyssey + headset.

​​​​​​Strangely both my motion controllers where working responding to joystick and trigger inputs. Does Unreal have a separate builtin VR viewer that works independently of SteamVR or was it using the SteamVR libraries?