As explained here, Internet Recovery does not delete your old data. Does Internet Recovery erase everything on my Mac?
But what specifically function of Internet Recovery that won't delete your data?
TL;DR I installed Catalina beta, and then used Internet Recovery to re-install Mojave back. Files gone, but system data is 100GB+
Here's what I did. I installed MacOS Beta (Catalina), and I realized I can't submit iOS builds to App Store with this MacOS (believe me, I did everything I could including trying different Xcode builds and so on, and this is the last option I could do, reverting the system to Mojave).
So from Catalina, I restarted my macbook with my fingers holding COMMAND + OPTION + R. And then I got into that rotating globe thing, and I connected to wifi, and I waited 1 hour or so for this. And then at the last part of this adventure, I got into an option of downloading and eventually installing Mojave.
After installing Mojave, all of my files are gone, I was not surprised, I was expecting this. But not this:
I feel like my old apps are somewhere in my storage.