You can open a STEP file in Autodesk Inventor (create a file) or place a STEP file into an assembly.
Imported data can be placed in a Construction folder that contains individual group, solid, surface, and wireframe nodes. Surfaces are contained in a single node to help manage them for promotion in Part modeling, so you can promote multiple surfaces as a single feature.
In many cases, composite features possess the same behavior as Base Surface features. However, they can also have multiple individual bodies within them, such as solid and surface bodies that are not stitched as one. You cannot interact with the individual bodies.
See Assembly Tools for more information about the Rename Browser Nodes command.