Fortunately, Pinephone and Librem 5 still exist and work quite well, despite being relatively slow. Sent from my daily driver Librem 5.
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they designed an elaborate process which theoretically guarantees that your phone was not tampered with at the factory or in transport. it was sort of a reaction to Bloomberg's "The Big Hack" article, which claimed Apple devices were compromised en masse at the factory in China by state actors (and the story was later retracted due to lack of evidence).
i do think it's a cool effort, even if the threat is only hypothetical. but it's a lot to pay unless you're operating under an extreme threat model.