How exactly you arrive at understanding the codebase is not as important. Just make sure you keep actual development separate from learning the tool if you car

How exactly you arrive at understanding the codebase is not as important. Just make sure you keep actual development separate from learning the tool if you care for your results and especially reliability. Now, I use various ways to learn the tools and codebase. Running PoC for my idea or maintaining separate toy project helps me with maintaining hygiene. For example, for the past year I have been spending a lot of time learning reactive programming with RxJava and Reactor. I have created a dozen small projects illustrating various ideas for making reactive APIs, processing pipelines, separating business logic from infrastructure, composing reactive modules, etc. I did this with aim of purposeful learning rather than writing production code, even though some of these in the end migrated to be part of production codebase. I am now at a level where I can, again, write large swaths of modules, libraries but now using reactive paradigm, with a very good chance of it working correctly, which for me validates that I more or less understand what is going on.