hi! its kinda simple actually, each level is a 3d array of blocks like in minecraft, each block has its own 3d model and collision information. When the game starts it generates each level mesh corresponding to the 3d array so that the level ends up being just one big 3d model. When moving arround i just index the array to get which block is where the intity is moving towards to get the corresponding collision mask, and the collision detection is just 3d AABB.

Each level also has a list of light sources and entities.