Lot of the big shop mechanics do side work at home, usually for cash.
My dad grew up with a guy who is a mechanical and automotive genius and all around good ole country boy.
He works as some consultant to heavy equipment manufacturers and isn’t really available any more. The guy gets called in when the people who designed and built whatever machinery can’t figure out the problem.
After asking some questions and getting whoever to check this and that plus drawings, he will say the problem is way over in some subsection that appears not to be involved. They will balk some then they try his solution. It works.
His brother is said to be even better. I didn’t know him well.