3D objects assets on 2D background

Pillars of Eternity uses this technique https://eternity.obsidian.net/news/update--79-graphics-and-rendering-

Similar tech uses Transistor and Jotun.

I would like to know when I place background image on quad, how to place 3d character, asset, animation in particular spot projected on 2D image?

How can lighting (lamps, probes) affect 2D image material?