I guess if you use the car as WIFI, then you will need a data plan and the car will be the hotspot for other users. To use the CarPlay feature, you will need to connect a wire for the 1st. time and will be prompted asked if you wish to use CarPlay, if so, you need to enter the code shown on screen; accept on car and phone and you good to go. (All that on IOs. Not sure for Android) If you connect via Bluetooth, you will loose access to Apps like WhatsApp, Wise and others, but still can play music or access contacts on the phone.
So far, it has been working perfect fine without any lag or loosing connection on my car MY20. Just a few times did not connect when I switch on ignition, so I put the wire, but after I switch on the ignition again, it connects again on its own.
On youtube has quite a few video instructions how to connect CarPlay/Android with MMI.