NOW, I am assuming that the installer will install into the Addon Manager\MSFS folder, and will just create a shortcut to the Community folder, which is what allows to see the scenery in the Sim.
That's exactly what it does, and in fact is what the installer tells you when the page that indicates the destination folder appears.
y question now is, IF the MSFS installer will install the Addon Manager twice (in my case duplicating whatever files), since as previously reported, I have already this Addon Manager installed for the GSX GS / Level 2 Bundle
Of course the installer WILL NOT install twice, and it won't create duplicated files and it won't even let you do the mistake of installing MSFS product in one Addon Manager folder and P3D products in *another* Addon Manager folder.
You might have noticed that, if you already installed any other FSDT product before ( regardless if it was for FSX, P3D or MSFS ), the destination folder for the Addon Manager is GREYED OUT, so you can't change it, which is precisely what is supposed to happen, so you won't install into different places.
The only thing that matters, which the installer enforces, is that all FSDT products must stay together, for obvious advantage of sharing of files instead of needlessly duplicate files and waste space AND to make the update process easier and faster, since you only have to deal with one updater, which will update all your installer products in a single go, regardless if they are for FSX, P3D or MSFS.
LAST BUT NOT LEAST, the same question concerns procedures on how to install this guy here -> FSDREAMTEAM - ZURICH AIRPORT for MSFS2020 as well.
I'm not sure I understood the question. You install any FSDT product using its installer. Zurich is no different.