Creating game-ready assets is a huge subject, and there are many different workflows.
Generally you create a single low-poly mesh from the high-poly ones, and unwrap it with unique UVs. You then bake all the material texture maps from the high-poly to the low-poly mesh for a single set of textures. You can also bake down the geometry detail from the high-poly meshes into normal and ambient occlusion maps.
If you are doing this yourself, I’d suggest looking up Blender tutorials on creating game-ready assets, retopology, UV-mapping, and texture baking.