The part which deals with all this is the battery management system, and it's almost always integrated into the battery itself, for reasons of practicality, safety, and seperation of concerns. Its interface to the rest of the car and charging system is fairly straightforward: mostly it just needs to report state of charge and current limits for charging and discharging (chargers for these packs are rarely anything more than a constant current supply being controlled by the BMS). It can in theory provide more detailed information but this is just diagnostic, not something the user or any other part of the system can really take action on (for example there's no way in current EV packs for the motor to draw from specific cells more than the others).

Basically the only way for car manufacturers to make this interface hard to replicate with third-party packs is by introducting some kind of DRM-like signatures on the messages from the BMS.