Branch: Use it when you have write access to the repo (your own project or a team repo). Create feature/login, work, and open a PR into main.
Fork: Use it when you don’t have write access, as in most open-source contributions. A fork is your own copy of someone else’s repo, where you push branches and then open a PR back to the original.

Keeping a fork in sync:

On GitHub, click Sync fork on your fork’s page, or
From the command line:
bash
git remote add upstream https://github.com/ORIGINAL/REPO.git
git fetch upstream
git checkout main
git merge upstream/main # or: git rebase upstream/main
git push origin main

Always create a new branch from an up-to-date main for each contribution.