[warkdarrior]

> How do you recover your keys if you lose your hardware? What happens if you lose your phone and have no extra trusted device?

You get a new device, create new passkeys, and re-enroll into the online service again.

> And, for Yubikey, how do you backup?

See above.

> Do you need multiple Yubikeys?

Yes.

> Do you need to manually make a copy of every keys? How do you know if the copy is synced with the main one?

Apple's and Google's solutions both sync the passkeys via their cloud services. You cannot sync passkeys to Yubikeys, AFAIK.