Serious question re: keys

I know I know “Not your keys - not your coin.” And I’ve heard the horror stories of lost keys and locked coins. But how do we know that the keys generated by the various wallet apps are known only to us and forever deleted? Could there be any data or history at the wallet app end that could compromise security? And how do we ensure that the algorithms that generate the keys are random enough that couldn’t be reverse engineered? How do we ensure trust in all these new platforms as systems as we scale? Honestly just wondering - not FUD. And don’t bother DM’ing me - I’m not even a shrimp ????.