Question about cad assembly file

Hi, i'm working on a robot simulator, and i need to insert the information about the joints of my robot into the program. For now i can only import the stl file of the pieces, but i need to concatenate them in some way, programming an importer to do it.(i can do only manually for now).

Can you tell me how the cad program work? What kind of assembly file they produce and its extension? (and if it is standard or not). If there is a simple-standard format for assembly, i could write a code to read and use it.
Step doesn't support the information i need?
Thanks!