Yup, and that is straightforward, if time and money consuming. Changing zoning isn't enough: making an area denser without appropriate matching infrastructure investment isn't going to go well. If you want the next Tokyo or Paris (or even something significantly smaller), you need Tokyo or Paris' infra.
Then the next problem is that people can't stand each other. People don't want neighbors because neighbors are a pain. It's a problem everywhere, but in the west, the US has it particularly bad. I don't know about Cali, but in a lot of cities the building code is a total joke, and Americans are known to be LOUD. City rules are rarely enforced properly, either. So the only option you have is to prevent neighbors from moving in in the first place. That's easily fixed: better construction quality and clearer, enforced rules that make coexisting easier and nicer, alongside investments to keep the area from going down the shitter.
Do that for a while, and people just won't mind. Having moved from a dense city in another country to a medium sized American metro made me "appreciate" why NIMBYs are such a problem here. Living near people can become a nightmare real quick if you're not lucky.