When the work on a branch is done, you bring it back with git merge. First you switch to the branch that should receive the work (usually main), then run git merge <branch>.

This level is the easy case: the feature branch is simply ahead of main; main never moved while you worked. So git doesn't need to combine anything; it just slides the main label forward to catch up. That's called a fast-forward.

You're on the feature branch. Switch to main, then merge feature in and watch main's label glide up to meet it.