It's all a bit of a mess right now, but with some fiddling in settings you should be able to get your passkeys in one place (probably Bitwarden) and access them everywhere.
Safari on iOS can store and use passkeys from any app that implements the right system API, including the default Apple Passwords but also Bitwarden and Chrome.
For desktop, you can either use a browser extension provided by some password managers (such as Bitwarden), or if you're on a Mac, Safari and Chrome can access passkeys from other apps similarly to on iOS (but not as many providers support this API on Mac as on iOS, and in particular Bitwarden doesn't, so you'd have to use the extension for that).
Safari on iOS can store and use passkeys from any app that implements the right system API, including the default Apple Passwords but also Bitwarden and Chrome.
For desktop, you can either use a browser extension provided by some password managers (such as Bitwarden), or if you're on a Mac, Safari and Chrome can access passkeys from other apps similarly to on iOS (but not as many providers support this API on Mac as on iOS, and in particular Bitwarden doesn't, so you'd have to use the extension for that).