What does mechanical engineering involve

Mechanical engineering deals with the designing, manufacturing, and maintenance of machines. Robots, airplanes, cars, heating and cooling systems, industrial equipment, medical equipment, it's very broad, probably the most broad engineering discipline. You need to be good at math and physics: thermodynamics, fluids, heat transfer, kinematics, material properties, statics, controls, etc.

If you mean day to day tasks, there is no single answer, I know some who do research/run tests, some who do maintenance, some who refine processes/design, and some who do a mix of everything.

It involves only Mechanical Engineering.