See the triangles.ts file in Explorer, that contains the icosahedron (20-sided die) definition used for the asteroids. It defines vertices (corners) in 3D space, and faces that join the vertices. You could modify that to draw cubes instead.
However, the code is quite messy and not really suitable as a general 3D engine. This thread contained a very early version which eventually turned into Space Rocks 3D.
For more geometry variations, see the joke variant Space Rooks 3D from this chess thread which has a chessboard and some rooks created as rotationally-symmetrical pieces: https://makecode.com/_aFKb1UFEjCyC
The latest version of the renderer is in the unfinished Carrier 3D experiment. That version has additional features, for example BSP tree models that don’t need to be convex. It separates models (3D shapes) and instances (copies of that shape with individual locations and orientations), but it’s unfortunately also more complicated.
Unfortunately it’s hard to make an easy-to-use 3D library, especially if it needs to be efficient. Something like OpenGL is already complicated, and recent libraries such as Vulkan or WebGPU need tons of setup code to even draw a simple triangle :-/