I've noticed a few iPhone apps have started to allow you to use your password manager to sign in. Slack is an example of this:
It looks like there's a link that's triggering a partial version of the share toolbar in Safari which lets you access your favourite password manager's widget. That's all I've been able to figure out so far though.
My question is, how have they got this functionality into their app? I'm primarily developing things using React Native, but if it needs a custom component then that's not a problem.