It's all proprietary code. But in theory you could unlock the full potential if not more. If you can recreate their software. Ideally then also open source it please.

What’s to stop someone just hacking it and unlocking it?

I assume that they try to make that fairly difficult.

I mean, modern cars are Internet-connected, have cell radios. If the vendor can maintain access to the car and can provide the initial trusted hardware, they can make it pretty unpleasant to modify the thing.

You also don't need a 100% solution to make it financially work. Just need to make the level of inconvenience high enough that the bulk of people won't do it.

It’s like they don’t realise they don’t own the car once it’s been sold.

They literally don't. This is nothing less than a war on property rights.

What’s to stop someone just hacking it and unlocking it?

The DMCA Anti-Circumvention Clause makes it a felony.

If you hack it warranty becomes void and you're maybe in big insurance trouble if there's an accident or your battery catches fire in an underground parking garage. Basically similar to how banking apps etc keep people from trying to use alternatives to Android on their phone. The boundary isn't technical, it's legal.