GitHub fork copies the current branch (normally main) of a GitHub project. It copies the git repo into your own account for your own purposes or to help the project as a whole.
git clone copies the git repo at the current branch (normally main) onto your local machine. This allows you to work on the project in your own environment and IDE.
git branch creates a new branch in the current git repo. This allows you to work on a new feature or bug fix without affecting the main branch.