Pull requests made by contributors can be approved or merged by maintainers after the review. There are different ways of merging a pull request:
Merge pull request: Merges all commits to base branch. It keeps all commits made in the PR as separate and merges them as they are, through a single merge commit to the base branch.
Squash and merge: Squashes all commits created in the pull request into one commit and merges them as a single commit (with the help of a merge commit) to the base branch (see this blog for details).
Rebase and merge: Rebases all commits individually to base branch (integrating changes from one branch to other) before merging. In the prompt message, the maintainer can provide a merge message along with comments(if any) and then press the “confirm merge” button.