Bitwarden master password and public server auth

I have what may be a stupid question...

How is it your master password is both used to decrypt your vault and used to authenticate with bitwardens public servers to acquire a copy of your vault/view it in the web app, but bitwarden can't use that password entry to decrypt the vault themselves?

(please correct me if I'm misunderstanding, as I use self-hosted vaultwarden for my server instead of the public ones)