If you want to get started quickly, I recommend that you first find this geometric accuracy specification, or interchangeability technique.
Then find the standard of several industry standard parts, such as Allen screws, and learn to draw a few pictures to have a preliminary understanding of drawing. Assuming you still have some basic knowledge of mechanics, find a book on mechanical design and understand the strength check theory of individual parts.
Screws, bearings, keys, shafts. Of course, if you want to be more systematic, you must learn theoretical mechanics in the early stage, and if you have a preliminary understanding, you can ignore it for the time being. Then, go to industrial control websites such as servo motors, stepper motors, and automation to find a few simple structures, and use 3D software to imitate them.
It is best to copy the structure of the equipment you repair. Finally, understand the knowledge of cylinders and motor drives, so that you will generally have confidence in applying for non-standard equipment engineers who are least sensitive to academic qualifications.
You've only graduated for 3 years, you can actually catch the last train, you won't be embarrassed to ask people at work, and in a few years, if you make low-level mistakes in materials and tolerances, it's not very embarrassing.