I’m trying to create an L-shaped 3D structure made by combining or extruding cube segments. In the first image, the geometry appears correct—each face is a clean square and there are no unnecessary diagonal edges:
However, in the second image, which was created by manually using BoxGeometry or extruding incorrectly, I end up with triangulated faces instead of clean quads:
How did you create these images? The first one is hiding the truth from you: all geometry is always made of triangle meshes. A quad is made from two triangles, a cube is made from at least 12 triangles. Always. There is no way around it.
The first image is rendered in a way that only shows the cube’s outlines, the second shows the actual geometry as a wireframe overlay.