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"?
This is not new. Say you like to surf website regarding cryptocurrency and blockchain, you will have more advertisement from those subjects delivered to you. You see it in youtube, facebook and other websites that has on-site advertising. So these few days some of us may be bombarded by advertisement from a certain ICO. These apps got your data from the phone. They took it free. Imagine with Datum you get to monetize it. Brilliant really.