From Settings, record the person speaking (or upload a recording) and the app creates a personal voice from it, so I love you sounds like her, not a machine. The recording is sent to the voice service that builds the voice and is not stored by us. Cloning needs a signed-in account.
Voice setup can take a little time. Until it is ready, phrases speak in a clear built-in voice automatically. The voice lives with the account — sign in with the same account on her device and it is there.