Google and Apple Partner on Covid-19 Exposure Notification API

"a. Can I turn it off?

Yes. The choice to use this technology rests with the user, and he or she can turn it off at any time by uninstalling the contact tracing application or turning off exposure notification in Settings."

Does this mean the device owner has to turn it off? Or is it opt-in? Also, can random apps turn this on again after it has been turned off? And how can the user not get this update at all?

I imagine you're walking down the street, your phone starts to vibrate like crazy and you think "is it an icbm" no this person across from me is contagious.