Random full-screen ads outside any app mean one installed app is showing them. Browser popups and 'virus' warnings are a different, easier fix. Both are solved without paid cleaners.
- 1Identify the culprit: when an ad shows, open Recents, the app behind it is often visible there.
- 2Check Settings → Apps → recently installed; uninstall flashlight/cleaner/wallpaper apps you don't trust.
- 3Boot into Safe Mode: if ads stop, it's confirmed to be an installed app.
- 4In Chrome: Settings → Site settings → turn OFF Pop-ups and Intrusive ads; review Notifications and remove spammy sites.
- 5Run Play Protect: Play Store → profile → Play Protect → Scan.
Tip: Never install 'cleaner' apps promising to remove ads, they are the most common source of ads themselves.