Personally, I just use obsidian notes. Its simple enough, uses markdown, syncs to my phone. I like to break projects/problems out into checklists. Helps keep me motivated.

I don't use the 'linking' feature between notes. The whole 'second brain' thing seems like something you do to make a neat screenshot of your note graph. I just use regular old folders like a file directory. My notes have gotten a little messy though.