I'm mostly self-taught, but back in the 1980's bikes were simpler and we all tended to reach for a manual and have a go ourselves. When I was a courier I was doing around 70-100,000 miles a year, and learning to do my own maintenance saved a huge amount of money.
Although I made a fair share of mistakes along the way, like the time I picked an engine up from a friend totally dismantled in cardboard boxes. I built it on my bedroom floor and then when I finished discovered it was too heavy to get down the stairs 🤣