I like Josh W Comeau's content, he has a lot of free articles, but his paid CSS course goes through step by step why CSS is the way that it is, worked great for me for understanding it all. My work paid for it though, that's why it's priced so high, but I'm sure he has discounts for individuals.

This should be a good start: https://nerdy.dev/cascading-secret-sauce

There are a lot of resources.

I am a fan of: https://www.youtube.com/@KevinPowell

Kevin is a no-nonsense no-hype educator. He will keep you up to date without a lot of engagement hacking.