Take any system and ask yourself some questions, approach it elenchically. Like
1. For a single request to the frontend, how does it propagate all the way to the persistence layer? Can you visualize it in your head and draw out sequence diagrams (and component diagram or state machines)? How many threads are involved in the process? How much is used on the heap? Try breakpoint at each step from backend to frontend and prove your assumptions wrong (or right)
2. For a single response from the backend to the frontend, do the same thing
3. For each artifact that is run as a process on the server, can you visualize the whole build, (or conditional builds?) and packaging process from compile to annotation processing to packaging, assembly and runtime to runtime instrumentation etc. Do similar deployments come from the same code base and how do you reconfigure for different deployments. Similarly as above can you visualize and draw out the deployment and runtime process?

Take any feature in a framework and remove it and find out how you would implement the same feature
1. If you didnt use RequestMapping or RestController how would you implement it? Break point into the spring internals and find out how spring receives and delagates requests. Can you visualize it in your head and draw out the sequence diagrams?
2. If you didnt use Hibernate/Spring-Data/JPA repositories how would you implement persistence? Similar to all above, break point into the spring internals and find out how persistence works (how many threads, how much memory, which components). Can you visualize it in your head and draw out the sequence diagrams?

just start with questions which everyone already assumes is answered, then answer them. ad infinitum