How to draw a 2d animation behind 3d world

I’m trying to recreate the effect from Metal Gear Solid VR Missions where the background sky was a 2d animation that didn’t change no matter which way you turned the camera and it drew the 3d level on top.

I was thinking is there anyway to do a depth mask to a render target of the level then blend the 2d animation over the 3d world using that mask so it appears behind?

How would this be done?