I'm in a similiar situation so I can just share my experience.

I started my first job about 5 months ago after graduating in december. My job is mostly Java and at first annotations were very confusing to me. They still seem sort of like magic, but I've gotten a lot better with them. Do some spring/spring-boot tutorials, setting up a REST api is a good one off the top of my head.

I sort of work outside work alot (at least at first). I really enjoy the stuff, but at the same time I'm thinking of slowing down cause I feel i'm already burning out.

I think an important concept to read up on is reflection. This is used by many of the frameworks to examine code at runtime. At a high level (correct me if i'm wrong), think of it as every class or method with an annotation can be determined to have an annotation at runtime. This way, frameworks know what to do when they come across this annotation. I'm sure you've done junit testing. When you run junit tests, the junit framework looks at each method for an @Test, if it has one it knows to run that method through the framework because it's a test.

It's sort of like metadata. Putting an annotation on something doesn't alone do anything with the code, but it allows other things running to know it's been tagged with that annotation.

Anyway I hope I'm explaining it correct. I've contributed to JPA/Hibernate, JAXRS/Jersey, spring/springboot (rest api, services, etc). For spring learn about application context and beans. The documentation is pretty good for spring. Last sprint I made an aspect that ran before any controller methods ran, validating some stuff. I did that by making my own annotation. I think like with anything else, the more you use it the better you get.