I'm not really sure what you are asking for. Colimits and limits are easy to compute in simplicial sets, because it's a presheaf category (as you say). But if you want "geometrical" intuition about simplicial sets (including "pictures" of joins, etc.), you want to know about the geometric realization functor from simplicial sets to spaces. In particular, the fact that geometric realization preserves colimits (it's a left adjoint), and that it preserves finite limits (essentially a theorem of Milnor).
The limit result is proved in the book by Gabriel-Zisman. (Goerss-Jardine mention the result in chapter 1, but don't seem to prove it.)
