Either one of those would require using a third party phone app as tesla does not have a native app for watches.
Thus, ANY (and all) apps that do this on the apple watch are both third party apps, and also require both the car and the device to have access to cellular connectivity.
It will work the exact same way the tesla APP works to open, start, etc the vehicle. It will NOT work if there is no cellular to both the car and the device. Whether the device is a apple watch with cellular, or an apple watch without cellular, paired with an apple iPhone. The requirement for both the car and the device to be within cellular connectivity will be present.
In practical terms, that means (for example) if you park in an underground parking structure without cellular repeaters, there is a good chance the apple watch would not open the car. Same if you go anywhere with poor cellular coverage.
There is no "watch as key" (phone as key equivalent) even though you can start / open doors etc.