They actually don't though usually unless they're looking for some specific function of a specific board. Presumably anyone working with these things as a job had either a class or book that more specifically taught the syntax of writing code with HAL, or GPIO, etc, without just looking every single individual thing in an enormous PDF and piecing together the entire structure of their knowledge by doing that. I have no problem looking through a big PDF if I need some specific timing routine or way of accessing a register inside of a specific board, but there is zero doubt in my mind there is 100 better ways of learning how to work with this kind of thing in the first place then combing through a giant PDF.
Seems like the book someone linked above is pretty solid though I'll give that a good look.