Picked it up during about 15 years as a welder. And I went to community College for a couple years where they had a shop program and learned to weld there.

That being said I could probably learn a lot by taking actual formal classes, especially with CNC these days. Everything I know is manual machining, which is a dying art these days. Most old machinists have either retired or died with their secrets. Under 40 and able to run a lathe or mill with any semblance of accuracy is a surprisingly rare trait these days.