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Ifixit gives fairphone 5 a 10/10 on repairability and maintanence
(www.ifixit.com)
This is a most excellent place for technology news and articles.
And you can prove that the binaries I receive are the binaries from the developer?
F-Droid has reproducible builds, so definitely yes. You can even check the exact code used to build every app on F-Droid.
The rest of stores, it depends. Every store has their own policy and the only one I recommend is using F-Droid or downloading directly from the devs' website or Github repo. There's an app to automate this process for updates too, so it's actually a quite good method.