How to add more mesh and texture

I'm very novice in custom content business. so I examine the designers' custom content about how it is done. This content caught my attention and I wanted to ask how it was done. I've done some experiments and I've only been able to add a mesh and texture. but there are two mesh and texture. how can we do it. u have trouble with uv map. so it's easier to divide into two or more parts. Thanks in advance for your help. sorry for the bad english.