Germany's smartphone tracker is open source (Apache 2.0) with a version available on the F-Droid repository. This is the sort of thing that encourages confidence in claims about how the application works.
Idle Words on privacy
If you haven't already had the pleasure, I can recommend reading any and all of Maciej Cegłowski's Idle Words
He had gone a while without very many additions but has made several new entries recently, including this one, about the radically shifted privacy landscape we live in today:
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