BMW Umsteiger – SIM cards in the phone

I am currently driving a BMW E61 535d, which runs out of leasing in just under 5 months and is thinking about changing to an A7. I haven’t had an Audi yet, therefore a question about the “Bluetooth Auto Telefon Online”, which I could not yet clarify: Both the phone and the separately ordered operator have a SIM card reader. What does this mean in practice? In my BMW I have currently connected an older Nokia phone firmly to the car, which has its own SIM card. Dam I can make phone calls and it also works all the online services of the car (Internet, Mails……). I mean to have heard that the BMW itself has an internal SIM card installed. Which SIM card reader do I have to use in the Audi in order to be able to use all the online functions next to the phone? Do I even need two SIM cards?