What is the most useful thing in mechanical engineering and how much subject are there

Mechanical engineering is a discipline of
engineering that applies the principles of physics
and materials science for analysis, design,
manufacturing, and maintenance of mechanical
systems. It is the branch of engineering that
involves the production and usage of heat and
mechanical power for the design, production,
and operation of machines and tools.[1] It is one
of the oldest and broadest engineering disciplines.
The engineering field requires an understanding
of core concepts including mechanics,
kinematics, thermodynamics, materials science,
and structural analysis. Mechanical engineers use
these core principles along with tools like
computer-aided engineering and product lifecycle
management to design and analyze
manufacturing plants, industrial equipment and
machinery, heating and cooling systems,
transport systems, aircraft, watercraft, robotics,
medical devices and more.The fundamental subjects of mechanical
engineering usually include:
Statics and dynamics
Strength of materials and solid mechanics
Instrumentation and measurement
Electrotechnology
Electronics
Thermodynamics, heat transfer, energy
conversion, and HVAC
Combustion, automotive engines, fuels
Fluid mechanics and fluid dynamics
Mechanism design (including kinematics and
dynamics)
Manufacturing engineering, technology, or
processes
Hydraulics and pneumatics
Mathematics - in particular, calculus, differential
equations, and linear algebra.
Engineering design
Product design
Mechatronics and control theory
Material Engineering
Design engineering, Drafting, computer-aided
design (CAD) (including solid modeling), and
computer-aided manufacturing (CAM) mechanical
systems. It is the branch of engineering that
involves the production and usage of heat and
mechanical power for the design, production,
and operation of machines and tools.[1] It is one