A mechanical contractor is a jack-of-all-trades. They can work on a wide variety of different projects in many other trades. If there are mechanical parts, a mechanical contractor can ensure everything is installed and working correctly. This includes large-scale plumbing, HVAC, electrical, piping, ductwork, and gas work, just to name a few of the trades they work in.
Mechanical contractors work in a supervisory role where they oversee projects and work with other mechanical workers to ensure the projects run smoothly. They may hire subcontractors who are specialists in their fields to help with projects they are working on.