I don't think that clients upload user keys to their servers. This would be really bad. Imo they live within the app on your device. That's why I wonder if the remote signer device is really more secure. Because at least hardware wise I guess modern phones are more secure than the remote signer device. But maybe I'm wrong, just really curious. Would immediately buy one if I knew that it really improves overall security.

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In a perfect world, they won't but just use a hash of them or something, but I didn't check client code (I wouldn't be able to read it anyway) so I don't know for sure. Of course this is overly paranoid thinking. But then again, we live in the era of vibecoding, lol.
The real risk is losing your phone or it being compromised by being a multiuse device. The signer keeps your nsec safe and sound from an internet connection you trust, and only does one thing so attack vector is 100x limited.