Behavior Trees are an incredibly powerful tool that can be used for building game AIs, but it can be daunting at first trying to learn how it all works.
Here are some resources that I have found very helpful in aiding my understanding of behavior trees.
Behavior Tree Introduction
This is an incredibly handy introduction video by the AI and Games YouTube channel that gives a nice overview of what a behavior tree actually is, how they work, and why you probably want to use them.
Chris Simpson's blog post on Behavior Trees
Chris Simpson, developer of Project Zomboid, has a really good blog post that will add on to the previous introductory video and really help cement in your mind the concepts that drive behavior trees.
Behavior Tree Lecture
If you are more of a visual/auditory learner, here is a great lecture that goes more into depth than the first video about behavior trees.
GameAIPro's Behavior Tree Starter Kit
This is a great document from GameAIPro that gets into how behavior trees work and also comes alongside solid code examples. This gives you some insight about how you might actually implement a behavior tree.
Behavior Tree Book
This is an extremely detailed book about behavior trees by Michele Colledanchise and Petter Ogren. If there are any gaps in your knowledge left from the previous resources, this resource should fill those gaps.
I hope these resources will help all of you out as much as they have helped me, not only have I learned a lot about behavior trees from them, but also AI in general.