For example, if I want to collaborate on making a (non-computer related) textbook with someone, and we want to use git as a version control system. Should we use github/codeberg for this or is there a platform that’s more suitable for using git in nontechnical contexts?
Any version control system works for this.
If you want something fairly user friendly, maybe look at Joplin or Logseq which are both specifically for text.