A UV map is a flat representation of the 3D model. To create this you need to unwrap the model. To unwrap models you need to select edges where the seams of the UV map will be. A dense mesh with millions of polygons will mean more edges that need to be selected by the artist.
You are probably thinking to yourself can this be automated. There are some methods for automated unwrapping but they are generally not as good or space efficient as the manual method. This can lead to lower quality textures or it can even cause issues when texturing. There are methods for texturing without unwrapping, but that brings its own issue and isn't very popular, in either the film or game industry
Here is a video that clarifies what unwrapping is (skip to 3min 20 for what unwrapping is and skip to 5min for seam selection)
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The reason you need low poly models for animation (apart from performance) is to ensure the model deforms properly. You need a clean edge where you want deformation to happen otherwise you can end up with weird artifacting when trying to bend parts of the character.