Hello, I just ran into this issue about a week ago. I will be using my phone normally switching in between apps and when I open (Random) apps a video advertisement will play. It will want me to watch the ad and then download it in the app store. Now when I say random apps I'm talking about ones used daily that don't usually have advertisements, i.e. snapchat, web browser, twitter...
Is there a application I can run to track from which application the ad is coming from?
You should be able to catch it via NotiStar. Try this:
Now in your lock screen you should see a double upward pointing arrow appear above the finger print icon. If you swipe up on that, it will open NotiStar and show you what pushed things to the screen.
You may want to go into your phone dailer and hit the three dots on the top right. Go to settings and them go to more settings. Stroll down to customization service and turn that off.