Not quite that complex. The FBX becomes the game engine ground (with a few controls added so you follow the surface when walking and don't walk through things) You don't just look at the your TG render, you explore it
Try setting up a TG render with an ortho camera looking straight down (nothing too extreme with displacement). Low quality render to export FBX (keeping polygon count down to a reasonable size), higher quality render for the texture. Bingo, one VR terrain.
The skybox approach is just an extension of that ... up close and personal.