I would go through the Springboot tutorials online setting up different types of projects, and then try start deploying your pet projects online.

https://spring.io/guides/gs/spring-boot/

You dont really learn the "magic happening under the hood" due to all of Springboots default configurations, but still i would start there because easier to get going, and then I would try build projects without Springboot (and use older Java frameworks) if you are doing so to prepare for a job that uses some legacy framework you want to prep for.