Bitwarden lets you extract and import passkeys. Apple, Google, Microsoft, and 1Password don't support that directly, but they do support a finicky app-to-app transfer system, where you install two password-manager apps on the same device, and you directly export your passkeys from one app to another. It's called the "Credential Exchange Protocol" (CXP).

Bitwarden's app can receive CXP passkeys, so you can install a passkey in Apple's password manager, CXP it to Bitwarden, and then export it to a file that you control.

Putting the file under your control does make it possible for someone to trick you into sending you that file, undermining some of the phishing protections of passkeys. It’s up to you to decide whether protecting yourself from being tricked into exporting your passkeys is worth sacrificing your ability to read them.