An interesting question. Looking back at my checkered career as a DIY (No, I don't mean Checker Cabs), I remember my first car, a 1934 Chevy, which I got about 1948. I was one year older than my car! I completely disassembled the engine and laid the parts all over the garage floor. I didn't know diddly-squat about good mechanics work at that time. I reassembled, using all the old gaskets and the darn thing ran! I drove it for a few years and then sold it for $25, still running.
Eventually I learned enough about cars to do a fair DIY job. Much was learned by taking things apart to see what made them work. Other times I found good information in magazines. Popular Science, Popular Mechanics come to mind. Remember Smokey Yunick? When I got really into wrenching, I'd buy a Haynes or Chilton manual. In those days cars were simple enough so that these books actually were a help.
In the late 60's the high school where I taught started a vocational mechanics course and I was able to both ask the teachers for information and to use the shop during vacations. Wow, a car on a lift was soooo much easier to work on!
I think what has helped me the most with more recent cars was my training as an electronic technician while serving in the navy. It seems that more and more troubles are electrical/electronic in nature. My first cars had valves, valve guides, and rings which wore out much sooner than today's cars do. Repairs tended to be mechanical. Over the last few years, I've done far more electrical troubleshooting and repairs than mechanical.
Questions like this start me remembering the many cars I've owned. I sure would like to have some of them back. For example:
1926 Model T Ford
1937 Chevrolet
1951 Studebaker Land Cruiser
1952 MG TD
1953 Chevrolet 2-dr
1954 Austin Healy 100-4
1960 Chrysler Windsor
1963 Mercury Comet wagon