I am another second brain and school type user. I am less heavy into the second brain part though.

My school notes are from my current program of study. I keep my undergrad notes in a separate vault. I find I rarely need to dip in there, so I keep it separate as not to clutter my main vault.

I slack a bit with the second brain, but it has saved my rear more than once. I follow a lot of Tiago can't remember his last name at the moment's second brain approach with some extra ideas I have collected over the years. I generally use it for: Reminders for where I stored items that might be important in the future that I don't regularly use. Interesting quotes I collect over time. My calendar archives. Lists of things I would recommend to people. How to manuals for things I will need to do that I rarely need to do but is still important. Receipt database for large purchases with warranties. A networking/contact list that reminds me how I know know people, traditional contact information, and important information that I might not remember long term but will be helpful for things like gift ideas, food preferences for restaurant suggestions, remembering the content of networking "interviews" and things like that. Brainstorms for future projects I might be interested in doing. Storing important information for current projects, and so forth.

I don't use linking as much as I should. The fancy connection charts are usually just for school.

I keep a pocket notebook and a part Bullet Journal, part commonplace book on my desk to dump information over the day. Every evenining or morning, I will copy over my days to do list and habits for the day so I can give some thought about what needs to get done, what to do list tasks that need to be trimmed, and help me plan my day a little. Once in a while (depending how busy/lazy I am), I will go back, decide what is important enough to keep and let the rest to languish in the notebook. The pocket note is quick notes and anything else that doesn't need to go into my to do list or calendar. If it's worth keeping, it either goes into the main notebook or my to-do list. I also do a weekly and a monthly check in to figure out what needs to get done this week/month. I admit, there is a lot of repetition in my process. Usually, I rarely spend more than 3-10 minutes on anything in the process, but the process makes me more mindful about what needs to be done and what is actually important. Most importantly, it allows me to search and pull up stuff quickly instead of flipping through page after page of the desk journal.