There's a difference between acting as an AI and training the AI. Training can take a lot more computer and data input resources, and/or time. The training results in a data set that can be downloaded to "reasonably" sized computer where it can act as an AI. There could be minor "learning" and calibration functions locally in the car that could make cars behave a little differently. But the main AI functions are trained outside the car and downloaded as part of the firmware.
It is also possible that local map data could be loaded in real-time and it could contain updated info or guidance that could make the car behave differently than before, even without a full firmware update. Something like "HD" mapping of each lane of a roadway might be updated to follow the lanes better than before.
I have no detailed knowledge of what Tesla is doing specifically.