Trezor uses a concept called "deterministic" public keys. From the private key, a solitary master public key is derived. From that single master public key, any number of extended public key can be derived. Each extended public key corresponds to a Trezor "account". For each extended public key, any number of actual public keys (i.e. bitcoin address) can be derived.
So, the private keys are only on the device, but either the master public key, or any of the xpub keys are transmitted from the device to both your web browser and to mytrezor.com servers. So, the private keys are not designed or intended to leave the device, and they are not required to generate new addresses safely.
So, the private keys are only on the device, but either the master public key, or any of the xpub keys are transmitted from the device to both your web browser and to mytrezor.com servers. So, the private keys are not designed or intended to leave the device, and they are not required to generate new addresses safely.
Thank you, that makes some sense. Going to research "deterministic" public keys...