Not sure what kind of popups all of you are getting but mine were as seen below. What everyone else was advising to do didn't work for me so I'll post my solution here. I'm on a Samsung Galaxy S9+ with Android 8.0.
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When the ad finally popped back up, I pressed the Recent button (next to Home and Back) to bring up the recent windows. I pressed and held the actual NAME of the ad window where the red arrow points to seen above (NOT the white space) which was "System" in this case. I had to press and hold on the NAME of the window otherwise the system would just say "Drop here for pop-up view". Then the same window gave me the circle with the i (in other words, the informational button) on the top right of the window.
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I clicked on the circle with the i and it brought me to the app info which (for me) was the Volume Control Widget app from the Play Store. It also changed the name of "System" to the app name Volume Control Widget app but only after I had already opened it with the informational button first.
Obviously now that I know it's the Volume Control Widget app that kept sending me ads, I've turned off notifications for it completely and any other settings that allows it to popup above everything else. If I still get ads, I'll probably just uninstall it and try to find another volume control widget app.
FYI removing one app at a time or "recently updated apps" is a terrible solution. Time for me to go back and reinstall all of my other apps, lol.
edit: Yep, even when turning off all notification and turning off "Apps that can appear on top" the ads still showed up. I've uninstalled Volume Control Widget app. :< Time to look for a new widget app.