How does AppLock by domobile lab earn ?

AppLock by domobile lab on google play has more than 100 million downloads but I dont see any add while using this applock .
I want to know how do they earn from their applock ?
Can anyone answer ? Please…

You don’t say if this app exists for iPhone users… I like the sound of it ))

It’s a free Android only app, which allows users to lock access to some specific apps and open them by using face or voice recognition. I’m not sure you can earn money on AppLock. Who told you so?

and speaking of how the App Lock profits, since this app is free, I think it was paid by Google.