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(post copied from SciOly discord; hopefully longer and more thought out responses can be given here!)
How do people start out learning new events, especially biology or chemistry events? I was looking at the rules for some events I'm considering doing next year but just reading the rules makes my brain numb. For example, I haven't even heard of half of the terms used in the Chem Lab rules, and have no idea how to start learning about them? For the es events you can usually just search up the word and you have the answer; most of the time there is no need for previous knowledge (for example, want to know what the definition of competence is? It's right there!). But when I search up the word "titration," I feel like there's so much background knowledge that I need to know in order to understand the concept. Any tips or resources are helpful!

Depends heavily on the specific event. For Chem Lab, you don't have to do the work yourself - you can just find a chem textbook, or online course, or whatever it is, and go through it for all the background knowledge (going shallower or deeper depending on relevance to the current topic). For a titration, you would definitely need to understand the chemistry behind acids and bases first.

Some events have a lot of overlap with school courses, and some don't. Chem Lab is basically a subset of AP Chem. Get a copy of Zumdahl and start reading.

In general, Unome's advice is good - get a textbook that covers the topic (could be one of the free openstax ones, or raid the library) and read for background. Khan Academy has some good intro stuff for some topics, too.

In general, I agree with your observation that the chem and bio events require much more in-depth knowledge and background than the environmental ones.

There are definitely some events that are better suited to a more senior competitor (just based on the background they will have acquired from their school courses.) If you have people on your team who have taken AP Chem, or perhaps are taking it this year, they are probably your top picks for Chem Lab.