This is what you are looking for: https://youtu.be/pA0zYTmi5ck

If buying new, I believe you can ask to have the modem removed from the vehicle, which wouldn't allow your car to access the internet. Haven't had the opportunity to try this myself yet, but very much plan on it for whatever vehicle I purchase new in the future.

You really can't

Also don't start breaking your own car like some of these comments suggest. It can go wrong in many ways and may even harm the value of the car.

Don't buy a Tesla or BMW. Done.

You can just not connect your car to any internet. Most casual brands have literally zero outgoing connections if you don't add or connect them to a network. Androd Auto and Apple Carplay are just displaying what your phone sends to the screen, the car itself doesn't access the internet through those. Think of android auto and carplay like "HDMI monitors for your phone that have touch too". Your phone does everything the car just displays it.

Connecting via bluetooth should also not be any problem since bluetooth doesn't include internet access (unless you activate that ok your phone but Im sure the car will not use it). Bluetooth only sends and receives small bits of data that your phone chooses to send, not what the car chooses. Contacts names, phone numbers, audio and microphone are the only few data that gets sent to your car and only during phone calls or audio listening.

In the end, just avoid cars that have always connected systems like Teslas or modern BMWs or similar cars. Most Volkswagen, Audi, etc etc are 100% offline cars when you don't connect them to a network. Most now can do it, but most its a subscription service that you can just not buy, and some even need SIM cards to work, that you just not use. Unless its a Tesla, those are connected even if you don't pay the subscription.

Test drive the car. Disconnect it from all networks or don't turn them on. Try to use all features. If the car constantly complains that it has no internet access for all of them, thats good.

Note that GPS access is always on and doesn't require any subscription, so maps and navigation will still work. However that is not really a privacy violation by itself because GPS on cars and phones only receives signal, doesn't transmit anything. You wont have traffic information or weather or anything tho. If you have traffic info, the car is connecting to some network, find how to deactivate that.

Many modern cars are too connected, thats true, but with the exception of a few brands, most cars go 100% offline the moment you disconnect them from their data services or don't pay for that upgrade/subscription. So you will be fine even with a modern car.

As long as you have a Google or Apple phone in your pocket... The car will actually not gather much more than your phone already does... So don't overthink it.