What happens when you sell the car? Does it go back to factory features?

I already said - But it’s gotta be a 1 time purchase and is lifetime of the car.

or you’ll need to modify the documents after unlocking the feature.

Usually when you unlock something it increases the value of the vehicle, and you need to let insurance know or the extra value won’t be covered. This is already pretty normal in any kind of insurance if there’s a change that can be considered material. If you don’t let them know, they won’t cover the cost of the altered/new item.