How do I find out which app on my android is malicious? I have root

[Philip Valkov]

I have these really annoying ads that pop up everywhere. Even on my home screen. I tried every single anti virus that I could find, not one helped me. So I decided to unlock my bootloader and restore my phone. I also rooted it. So, I restored all my photos, music and apps (only apps from the play store! And really famous ones, before I had some apps downloaded from the internet, now I only reinstalled ones from the play store) and the ads were gone. For one week. I removed some of the system apps I don't need and after one week *surprise surprise* tge ads are back. My question is, if this malicious app is installed on my phone as a system app, how do I find out which one is it to uninstall it? I used ROM toolbox to look at all of the system apps but they all look like default android apps, widgets and etc...