I underline and write in the margin, using my own symbolic shorthand for common words. I list all the symbols on the title page so I can decipher it in 5 years

I underline and write in the margin, using my own symbolic shorthand for common words. I list all the symbols on the title page so I can decipher it in 5 years. At the end of a chapter, I go back and make long-form notes in markdown, then commit to a git repo.