It's called an "incorporation service" or "incorporation consultant". They help you "incorporate", or form a business officially and legally.
The legal aspect is the primary focus, since this is what most people need professional help with. But many firms that do that also provide some services similar to those you describe (they would call them "logistics" and "provisioning", "promotion", or "brand-building" services, depending on the specific goals you want to accomplish)
Services such as Rocket Lawyer, Legal Zoom, and many others that provide these service describe themselves with these terms.
The correct general term for one firm providing help to another firm in running the second's business is "business consultancy" - which covers everything mentioned above.