Usually it is the same as any other projects and their code base.

One by one - merge the first, a Major change or important Bug, then ask the other issues' resolvers to update their fixes / patches / code changes.

Merge the next change, solve the next issue, if there is code conflicts again repeat point 2. and then 3. again.

Usually, it is the same as any other software projects and their code base (outside Drupal).

One by one issue - merge the first, a Major change or important Bug, then ask the other issues' resolvers to update their fixes / patches / code changes.

Merge the next change, solve the next issue, if there is code conflicts again repeat point 2. and 3. again.

Publish the official new Drupal project (module) release version.