Spring framework may look complicated only at start, until you get it, but then it becomes quite easy to reason about.

OTOH, Spring Boot is a huge pile of various loosely coupled framework connectors, web, queues, security, databases etc. Some of them are of good quality, some are not so good. It is that uneven mix giving the perception that Spring is a mess.

Agreed, Spring Framework is quite simple... just look here - https://vectree.io/c/spring-framework