RESTe2ePostmannewmanPostman is a great tool to explore REST APIs. You can build requests and try them out to get quick feedback. Then you can persist them as collections to make sure that the knowledge doesn't get lost.
KotlinJSONJacksonRESTSpringBootBackends that provide a REST API or consume other APIs have to deal with JSON quite a lot. Luckily combining Kotlin and Jackson makes it a breeze