TL;DR: A copyright notice is just the list of the copyright owners. It does not give magic licensing powers. The “… and contributors” is a truism: each contributor keeps their copyright unless they sign it away. This is the same as the absence of any copyright notice.
A copyright notice is not required, but is strongly encouraged. If you have a copyright notice, it should list the correct copyright owners. If a project was started by a single author who later incorporated contributions by other authors, the primary author does not own sole copyright. For projects with lots of small contributions by many people, it would be unfeasible to list all actual authors in the copyright notice. Instead, it is common to list the group of authors as the copyright owner(s), or to explicitly list the primary authors and note that additional authors exist:
Copyright <year> The <project> authors
Copyright <year> <primary authors> and contributors
The complete list of authors could perhaps be in an AUTHORS or CREDITS file, or implicitly in the version control history. But as far as I know, keeping a list of copyright owners is not required, it's just a very good idea to ensure traceability.
If the copyright notice would only list the primary authors, this would indicate they own the complete copyright for the project. In the example you cite, the copyright notice was amended to clarify that this is not the case: every contributor keeps their copyright. That is the default. If copyright were to be transferred, this would require an explicit contract (copyright assignment, usually as part of a contributor license agreement).
Note that some jurisdictions separate copyright from moral rights. Then, copyright only covers economic aspects: the copyright owner may issue licenses. Moral rights cannot be waived in some jurisdictions, and e.g. cover the right to attribution. I.e. even if I sign away my copyright to a contribution by accepting a CLA, I may have a right to be credited as a contributor. The copyright notice is not the place for such a credit.
A specific formulation of the copyright notice does not affect how contributions are licensed. Instead, this would be covered in a contributor license agreement or in the license itself. Notably, the Apache License 2 contains a CLA, and copyleft licenses such as the GPL family limit how derivative works (and therefore contributions) may be licensed. Also, the GitHub terms of service contain a default CLA, but that only codifies the expectation that contributors issue an implicit license when they explicitly submit contributions to a project.
