JEE. It's a tangled mess. I have no clue how to learn it, so I still don't get it.

It's also the main reason why I hate Java backend.

You realise no-one has used that in eons?

Pretty much everything these days is Spring, or if it's not, it's one of Quarkus, Micronaut, or Play. I haven't seen raw J2EE code in about a decade. Either way I pity you for having to learn Java that way 😅

JEE is obsolete. Something like 80% of java projects are made with Spring Framework but there are lots of other depending on your use case. Play framework, Quarkus, etc...