No, it's generally directly from main battery pack when the car is turned on, or when the car is charging. I would guses some models can also charge it now and then when the car is off, since some models are always connected to a cellular network.
> but then it seems more sensible to me to do away with the 12v battery all together.
No, no, no.. You need 12V when the car is off (for talking to key fob at the very least), but you probably don't want a DC-DC converter running from the main battery all the time. If you have trouble getting power from the main battery, it'll be hard to diagnose when none of the electronics work anymore. Having a redundant battery is a very good idea.
It's also a redundancy in case of emergency (some cars have buttons you can press to call emergency services).
I seem to remember Elon talking about removing it (of course), but they haven't yet, and I doubt they ever will.
I don't get this. If the converter stops working the 12v battery dies and your car stops working. I don't get why it would be easier to diagnose if it only charges when on. I assume you're talking about the converter charging the 12v battery? Even if you aren't it would still be simple enough to have some 12v points to supply external power.
Further is it redundancy or doubling the chances of failure? You now have a 12v charger that could stop working, and a 12v battery that could stop working. Both of those break the car. If the main battery goes that's basically bricking the car and you aren't going anywhere anyway
I admit it seems more sensible to charge it direct from the battery pack, but then it seems more sensible to me to do away with the 12v battery all together. So if you're going to have a 12v battery it may aswell be completely independent of the main battery pack.