: There are a lot of ways to do it. There's the method you linked, of course.
: Having a texture or a few textures mapped onto the inside of a cube or
: sphere is kind of the "traditional" way to do it, and you can
: choose between having it be real geometry that's really in the direction
: it looks like it's in with fake distance and depth or having a sort of
: surrogate that's mapped onto/into that area-- this was how Half-Life (and
: maybe other games in the series?) did its skybox, it would have a (very)
: small room in some inaccessible place in the map geometry (or outside of
: the geometry) that had the appropriate texture(s) applied to its insides,
: which would be mapped to the skybox, so you could just change the
: texture(s) in there to change the skybox, and IIRC if you used noclip to
: get inside it the graphics would get glitchy. Along with any of those, you
: can have flat objects in front of the skybox with the point generally
: being to add an illusion of much more depth while actually having only a
: little more depth (a 2D object has no depth at all, actually, but two of
: them layered together do). Having these distant extra 2D objects became
: more feasible along with having simple 3D objects way out there so the
: graphics engine can use most of the same tricks that make the main area of
: a map look three-dimensional to add depth to the background.
: I think so anyway, someone could come and correct me and that would be fine.

texture cubes are most likely used for the 3D sky boxes along with some other models. There a texture cube hidden inside the carrier models in new Alexandria is assume that it is a similar method to that used to project the sky boxes i'm not 100% sure though.