Those are semitransparent blocks, you can see through part of it, but not the “frame”. It doesn’t take much to make it more or less transparent. It’s a texture over a vertex grid. You can make things semi transparent without adjusting alpha just by not putting a pixel in the texture.
Maybe I should be more clear, no pun intended. The clipping mask and the texture layer are separate things. One object, that you can’t enter, the negative space is defined first, then the texture is applied. Sometimes, in games, those textures are applied simultaneously (within fractions of a second during recall -loading-) but they are still separate objects. This is how texture packs work. So the object, the actual thing, is by default transparent. The face we see, is just a image slapped on top of it. Voxels, or volumetric pixels, allow for the creation of both objects through their ID. Like painting in a bitmap image, but that image addresses X Y Z instead of just X and Y.