I think Apple (iOS and MacOS) is still the only platform that has a pretty decent implementation.
Google, I wouldn't trust them. Google Password Manager decided to nuke my passkeys for no apparent reason:
https://redd.it/1fkuwb7
Bitwarden is pretty good now for a cross-platform solution. But consider that since you're now storing your passkeys in a service which is automatically synced to all your devices, on the surface this seems to be less secure than a traditional password manager + 2FA authentication app. If all my passkeys are in Bitwarden, now if my Bitwarden account is compromised the attacker immediately has the passkey and can log straight into services. Before if my BitWarden account were compromised then they would also need to steal my physical phone where my 2FA authentication app was, because those tokens were not automatically synced anywhere. You can probably try to replicate that with passkeys, but pretty much every service is pushing you to sync between devices automatically.