Ok so I figured it out with some help, that the game juste creates a NTFS jun point and that was why it took 69GB of space virtually and forwarding it to E:/ where it's installed.
Like to keep it simple, all Appx program "live" in C:\ but use junction points to be able to move the all the folders to another volume.
But now I just found out I have PSO2 4 TIMES ON MY PC :'D
So now, I have the "100B7A24.oxyna18.104.22.168_x64_wyfsmff9ynw7j" folder in C:\Program Files\WindowsApps (11GB), another one in C:\Program Files\WindowsApps\Mutable (70GB) and finally one in E:\WindowsApps (10GB).
AND I also have the game in E:\Program Files\ModifiableWindowsApps\pso2_bin
I don't know even know what to think anymore
EDIT : Now I know, in short. You have 2 NTFS junction points if you install it on another drive than C:/ (or if you install in C:/ too anyway ?)
One for the launcher, and one for the actual pso2_bin folder.
In my example for the launcher you get the NTFS junction point in C:\Program Files\WindowsApps\100B7A24.oxyna22.214.171.124_x64_wyfsmff9ynw7j and the actual E:\WindowsApps\100B7A24.oxyna_126.96.36.199_x64__wyfsmff9ynw7j folder when first intalling the 10GB from the Microsoft Store.
Then you get the NTFS junction point for the actual full game in C:\Program Files\WindowsApps\Mutable\100B7A24.oxyna_188.8.131.52_x64__wyfsmff9ynw7j and the actual pso2_bin folder with the launcher you're actually launching when you've finished installing everything and launching the game from the Start Menu + all the data in E:\Program Files\ModifiableWindowsApps
So here you have it. And I wish we could delete 100B7A24.oxyna_184.108.40.206_x64__wyfsmff9ynw7j junction + folder when everything is set and running but huh...