> AFAIK android will have an open API, iOS however is going to only support iCloud, classic monopoly.

The Android implementation allowing for multiple third part passkey providers (typically, existing password manager that want to support passkeys) is in beta.

Apple does not pre-announce features, so we are all likely in the dark on their timelines, and whether their timelines extend to infinity.

However, I believe a passkey manager can replace the system implementation via the web extension mechanism in mobile Safari today.

You can use passkeys from a FOSS client on Linux, you can even use your Yubikey if you have one.

just because there are APIs that a FOSS client can call, that doesn't guarantee interoperabiltiy.