They are blocking the people using the adblocking features. But normally they have to show a message, I think YouTube is testing this "feature".

Mullvad Browser sue Firefox as base not Chromium

You can uninstall with adb

I know. You can also remove the apps only with adb, this is not the answer. The answer is what actually those apps do exactly?

No it's not a Facebook frontend they are without graphical interface and they don't need of Facebook app to work. The problem is, what they do exactly? And why Facebook need to puts 3 apps in your device and not just one?

I removed those apps in a old device and I find that they connect to Facebook very often, to send I don't know what.

Those are system apps in the OnePlus new devices but there are even in other devices like Xiaomi, Redmi, Oppo, Motorola and even some Samsung devices. You can remove those app with adb but I'm curious to know what they do exactly. In most devices they are hidden but you can see in your phone using AppManager from fdroid or using adb itself.

They sell even an antivirus, I don't think it's trustworthy company. In my personal opinion nowadays antivirus are a scam.