I know exactly what you're talking about, and I've been wanting to make a tutorial/educational video sharing what I learned, but I'm simply not getting around to doing it.Briefly, the issue that is even on a cube, each side is not associated with a separate texture.
Take a cereal box, cut along the edges and unfold it and lay it out flat on your kitchen table. Now draw a large square around it. That ONE square is the texture map for what is actually a six-sided cereal box.
And that, in a couple of sentences, is the essence of UV maps and unwrapping.