Different repositories name their primary development branches differently (e.g. main, gh-pages, master, etc.), and scripts meant to work with multiple repositories need an automated way to discover the current repository's default branch name.
What git command returns that name?
git rev-parse --abbrev-ref origin/HEAD will print origin/<default-branch-name>.
If the origin repository changes its default branch name, then git remote set-head origin -a will retrieve the new default branch name.