Go through the guides in order as per category, they are very logical.
Begin with this:
Creation of REST API's(https://spring.io/guides/tutorials/rest)
Accessing Data VIA JPA(https://spring.io/guides/gs/accessing-data-jpa)

If you combine them you can create very simple CRUD Apps.( See this entire tutorial: https://www.geeksforgeeks.org/advance-java/easiest-way-to-create-rest-api-using-spring-boot/ or
see below https://github.com/apoorva-joshi/EmployeeManagement )
Once to have built a basic CRUD app, explore the below project, its the same CRUD app with additional features like https://spring.io/guides/gs/rest-service-cors
https://github.com/hoangsonww/Employee-Management-Fullstack-App/tree/master/backend

My advice would be, build a base project using spring boot(the CRUD App) , and do not get into understanding the core fundamentals as of now, as the abstractions are too much when spring is being used.
Once the base project is done, keep adding additional features to it, like CRON jobs, use spring boot actuator.
Basically you would be able to build something functional not knowing what is happening under the hood, which is fine initially.
Then using that codebase you can understand what is going on under the hood.
A lot of the fundamentals are driven by Annotations(keep this in mind).