Adding a passphrase to a Trezor: what does it do?


I heard people talking about how adding a passphrase adds additional bits of entropy to your wallet.

It's pretend I have a trezor wallet with a passphrase on it and someone sees my seed. Can they generate the wallet without the passphrase and access funds?

Bonus question: If I have a wallet without a passphrase, can I add a passphrase to this or do I need to generate a new wallet and transfer funds?