Passkeys on iOS and macOS actually work quite well in that regard. They get stored in your provider of choice across the web, web views, apps etc., at least in my experience.

Mine is Bitwarden, and that's available on pretty much all platforms, natively where available (except on macOS currently), as a browser extension otherwise.

For the rare instance in which I need to authenticate using a passkey on a computer where I'm not logged into Bitwarden, there's the cross-device CaBLE flow where I can scan a QR code with my phone and use Bitwarden to authenticate. This works across OSes and browsers.