What are some specific things that mechanical engineers are responsible for designing?

Nowadays, a lot of colleges offer mechanical and aeronautical engineering together. Mechanical is an older engineering; hence, it covers a lot of topics that some engineers specialize in. As a mechanical engineer, you cover things in electrical, aerospace, civil, and, to an extent, material science engineering. Because of this large overlap, mechanical engineers have a wide variety of fields to work in. They can work in robotics, computers (hardware), civil engineering projects, and in flight/space design. Note, however, that because there are majors in electrical engineering, civil engineering, etc., mechanical engineers may not be the preferred or most specialized professional in many fields.

mechanical engineers design aircraft and auto structures and panels, and are involved in the mechanical design of generators

Mechanical engineering is a very broad discipline. It basically covers design of anything "mechanical." If it's not electronic, chemical, or biological, then it's probably designed by a mechanical engineer (Physical structures like bridges and buildings are done by civil/structural engineers, though mechanical engineers will design plumbing and HVAC systems). At a car company, they may work with the designers to make sure the body is aerodynamic, they may design the transmission and other moving parts in the engine, or they may be working on some other part like the seat belts or the air conditioner.

Nowadays, a lot of colleges offer mechanical and aeronautical engineering together. Mechanical is an older engineering; hence, it covers a lot of topics that some engineers specialize in. As a mechanical engineer, you cover things in electrical, aerospace, civil, and, to an extent, material science engineering. Because of this large overlap, mechanical engineers have a wide variety of fields to work in. They can work in robotics, computers (hardware), civil engineering projects, and in flight/space design. Note, however, that because there are majors in electrical engineering, civil engineering, etc., mechanical engineers may not be the preferred or most specialized professional in many fields.

mechanical engineers design aircraft and auto structures and panels, and are involved in the mechanical design of generators