I crack open a book about the topic (usually from manning publishing). Self-document my learnings in the form of tree-structured bullet-point notes. Usually in a git book space.

I can always revisit the notes to quickly refresh myself on the topic (if a context-switch happens later down the line, or if the topic is not related to my day2day work).

I've successfully used this approach to onboard myself for a Spring project (known to have a steep learning curve). It was my second time using Spring, with an 8 month time separation. The ramp-up/refresh took about a week.

There were also some positive unintentional side-effects of my approach. I keep my gitbook notes public. I found that making my notes public ended up helping others on other teams in my company. Specifically, more experienced Spring devs who were new to Webflux were able to use my notes to jump right into a client's code base.