Implement mini game inside game

Hello. A have seen tons of such topics but NONE was answered. There is some dude that comes and tells about some mythical control of a character on another level, but he never says how to do it, and google doesn’t know it as well.
Don’t say me to make a mini game in some not accessible location inside the main level since mini game has entirely different game mode and procedurally generated world.

I know I have to use 3d widget, thanks.

This is a really vague question. Are you looking to implement some sort of minigame like in the Mario Party-series? Or is it more like the terminals in the later Fallout games? You could simply change the level in the former case, and probably get something going by rendering to a texture in the latter.

I have a 3d endless runner game and I want it to playable from the terminal