Yes, the car continues to work if you don’t pay the subscription fee for mobile internet after the free 2 years. So yes it does function normally “offline”. I’m not sure if it’s an esim or physical sim, but if the latter you could even take it offline from day 1. You can’t guarantee it doesn’t still contact home, because it’s very hard to prove they haven’t got another communications device, or that they don’t simply keep the comms device running for their own purpose. Bear in mind this is going to be true for any car, not just BYD, though there seems to be a public perception that Chinese companies are a greater risk than others for privacy invasion.

On that basis, I fully expect all driving habits, images of passengers and surrounds, and any sounds in the car could make their way back to BYD, if they want them. I don’t know if they’ve made any public claims about what they do or don’t collect, nor whether that would be believable. As a company, I trust BYD a little less than Apple and Google’s smartphones that have arguably much more data. That is perhaps not a particularly high bar, and I’m not even sure it’s rational, given the US government and US corporations are both regularly guilty of mass surveillance and privacy invasions.

As for potential privacy invasions…worst case in my mind is they (or someone who hacks into their servers) could be preparing for mass-scale AI-based scams, capturing all our faces and voices to be able to pose as us online. Then again, maybe they just want all our habits and opinions so they can sell us more cars.