mostly through https://owner-api.teslamotors.com/
for example:
POST /api/1/vehicles/{id}/command/door_unlock
response:
{ "reason": "", "result": true }
Now, I personnaly wouldn't give credentials of my car out to folks, but you have to admit there is some interesting data that has come out of it.
for example, folks have discovered the details of how various tesla batteries charge:
https://forum.abetterrouteplanner.com/blogs/entry/29-faster-...