Well, Linux is 32 years old; GNU goes back to 1984, and Unix all the way back to 1970! The history of this OS is much older than Linus Torvalds's involvement; he "only" created and maintains the most popular kernel.
But yes, happy birthday to Linux. Many thousands have contributed to making this operating system what it is today and they all have my utmost thanks for it.
I know Okular can do at least most of that. Don't think it's available for Mac OS X, though.
Depends on a few factors, AFAIK as a non-lawyer. If the license allows closed-source derivatives (i.e. is permissive rather than copyleft), then anyone can create a closed-source version with all of the contributors' changes, including the original maintainer. And anyone can choose to keep it open-source. The community contributions still to some extent belong to the contributors, though the license waives most of their rights.
Some projects are copyleft, but contributors are required to sign a license agreement (a CLA) which allows a single entity to change the license as they desire, including to closed-source - this is a good reason to avoid such projects. The contributors don't own their work in such a case, but they can still fork the old project as it was before being taken closed source.
In a copyleft (e.g. GPL) project with no CLA, it's illegal for anyone to make a closed-source version, and a major contributor could sue even the maintainer for doing so.
In all such cases, the change to a closed-source model does not erase the existence of the open-source code with community contributions. A fork is always possible.
F-Droid is the more valuable app store anyway. I always check there before Google Play.
VOY: "Tinker Tenor Doctor Spy" comes to mind.