For me its mostly practical hands on experience. Like I had to re-thread the the oil feed port on my old SR which is how I knew what it was.

Also there isn't 50 million bolt sizes used on the car, theres about 8-10 sizes common through out. Its just like looking at the head of a bolt and knowing if its 12mm or 1/2, the same goes with thread sizes.

I also used to spend a lot of time researching and discussing how to make the car set up as best as possible. So I've read a lot over the years and had plenty of discussions about it with lots of people.