I want to learn backend, and just learned about headers and codes and response/request. What is the easiest framework to get into backend dev?

the easiest is probably spring because there are a ton of resources since it's the most popular java framework. other than that #ktor is tailor made for kotlin and there are also great guides available.

To learn the concepts, I'd be inclined to start with http4k. Way too much magic in Spring. In http4k you just have a function that takes a request and returns a response, it's a very simple mapping to what's actually happening in an http conversation. And it's all synchronous do you don't need to worry about async complications.

fair

I agree with Rob's recommendation, but would also add that Ktor is another good choice for a beginner. Ktor has good documentation on getting started, and one can develop a basic backend (manually - aka Explicit Development) without relying on code generation, reflection, or annotations.