How do technicians at dealerships and mechanics know what to do for major car repairs without potentially messing something up?

For big, complex jobs there seem to be so many steps and details and different parts, just accessing whatever it is you're trying to repair can require several steps. And cars are more complex now than ever with various computers and sensors.How do they know what to do step by step? Are there official guides for these kinds of repairs that they use (besides Youtube), maybe something put out by the manufacturer. And if so, can we mere mortals access such information? thanks. via /r/cars http://ift.tt/2tOHUIC