Need help learning about Texturing and UV Mapping.

Hi all.

I need a point in the right direction.

I started teaching myself 3D modeling again after many years. I’m currently making low-poly models to export as .md3 for gaming.

I made a model and learned how to apply textures to materials and then assign materials to individual faces of a mesh. These textures just happened to line-up perfectly on the model. When I go to UV Unwrap all I can do is export a stencil of the mesh with no texturing from the materials applied to the faces. Is there not a way to take a textured model and export a UV Map image based on the materials applied?

Where do I go to learn more about how UV Mapping works?