NOTE - This method is not for removing in-app ads, those come with the app.
But instead, these are methods to remove ads that show up on your home screen without your permission.





ADS IN NOTIFICATION BAR

If the ads are present in the notification bar, and you can't seem to swipe them off. All you have to do is long press the ad, and it will show app info. Go to app info and you will see the app causing the ad.
just uninstall it and your ad free.


ADS ON HOME SCREEN

If the ads are popping up on the home screen and other apps you can
1. Go to settings>apps>app setting>special access
2. There you will see a permission labeled "draw over other apps"
3. All you have to do is make sure only trusted apps have this permission.
4. Uninstall the untrusted ones, that's it.


ANOTHER METHOD

If the above method is not working you can install an antivirus application from the play store. I found Kaspersky useful. All you have to do is, do an in-depth scan. it will show all untrusted apps. just uninstall them and your ad free.

Kaspersky -  https://play.google.com/store/apps/details?id=com.kms.free