Oh that part's easy.

Money.

California has economic power over the rest of the US because of the money that flows through it.

As they said, money, but I'd say there are a few safeguards. On obvious issues like reproductive rights, we can hold out on "states decide" until there's a federal ban, and even longer with legal challenges. States that rarified it in their construction have a similar setup. We also have organizations in place to help undocumented immigrants so much so that they can get drivers licenses and insurance, etc.

As for things specific to Trump, it's mostly a matter of having a lot of lawyers ready to challenge things and purposefully not following federal law if needed. 4 years is a remarkably short time to hold out, assuming again they don't just go full fascist and arrest our governor and state Congress. Technically shouldn't be possible even with the supreme court but who knows.

Speaking of which, that's the real problem-- federal appointed judges. So all together we'll just have to see. States rights is a good thing when you've got a good state, though.