What is the technology that allows data from one app on your Android or iPhone to be collected by a 3rd party app? Aren't apps isolated from each other?
No not really, for example there are tracking features wich allow to control other apps. Further everything is (temporarily) saved on the smartphone/pc. The app just needs access (;
I did not know that.

Are you saying that potentially any app on a smartphone could have been eavesdropping on activity in other apps? Or what do you mean with "just needs access"?
in any application when you installing there are points where it's said that this application can use data, for example - contacts, access to sms, camera and other functions of your smartphone