There seems to be two schools of thought for "second brain" stuff:

1. Those who kind of semi-obsessively write down every little thought, semi-obsessively catalogue everything, etc.

2. Those who use the idea of "writing helps you think", where the second brain functions in support of this

Personally I find 2 quite valuable, and although I could "think about programming all the time" (or one of my other interests), I find it helpful to have f.ex. exact quotes from some book at hand (and various other Obsidian-related things)