Just to key in on the overlap between FOSS and privacy, because the source code for the software is open, it means that anyone can take a peek at how everything is running under the hood (among other things). It becomes possible to verify that software is storing data locally and properly encrypting when applicable (as opposed to blindly trusting the software's author and or lawyers).

It may also be a fun fact that best practice in encryption is to open source your algorithms. The helps safeguard against backdoors and mistakes/ errors that could compromise the security of the algorithm. Much for similar reasons as above, as it allows the security community to check your math (in a field where it is incredibly easy to get your math wrong).