Passwords can move between walled gardens generally very easily (export) or manually in all cases (enter by hand).
Passkeys only very recently got relatively broad support for migrating data (after years of promise and no support at all), and they report (optionally with hardware attestation) what password manager you're using so sites can force specific ones.
I can write my own authenticator implementation in a few hours by hand, maybe 5 minutes vibe coded. https://www.rfc-editor.org/info/rfc6238/
Even if you throw your phone into a volcano and buy a new one, you can still receive SMS verification.
KeePassXC supports passkeys
Passwords can move between walled gardens generally very easily (export) or manually in all cases (enter by hand).
Passkeys only very recently got relatively broad support for migrating data (after years of promise and no support at all), and they report (optionally with hardware attestation) what password manager you're using so sites can force specific ones.