“What happened to California that turned it from Republican to Democrat? “
A number of things working together.
Flooding the state with poor campesinos back in the 1986 amnesty worked its magic. They gained the right to vote 10 years later, and surprise, surprise, they like to vote for the party that promises a lot of free stuff.
The Silicon Valley has a population opposite that of poor campesinos, but they like to vote Left.
Losing the aerospace industry to other states hurt, too. Those defense engineers were once a backbone of Orange County’s conservative base.