I purchased MSFS via STEAM.
After completing the installation with all defaults, I found that all the 100+ GB files are stored under the folder
Users/<username>/AppData/Roaming/Microsoft Flight Simulator

Is it strange that the main folder is put under AppData/Roaming, but not under Programme Files (x86)Steam/steamapps/common like other games ?

kklee6

In this case, not really.   MSFS is really an application with its own launcher hooked in with the Microsoft store.   The Steam installation doesn't change that -- when you buy from Steam, you're just buying a Steam application that launches the Microsoft launcher, which manages the application.

AppData is a standard data directory for Windows applications, and MS uses it for a lot of their own stuff, so it's not unexpected they'd use it here.   What is a little odd would be a hundred gig of data in your Roaming folder.   Typically, the Roaming folder is used for data that you want to come with your account when you log into different machines with Active Directory, whereas Local and LocalLow were to be used for temp data or data too big to be practical roaming.

I moved the install location tore to my dedicated Flight Sim SSD, and for me the bulk of the data is in AppData/Local/Packages with a directory junction from there to the install location on my FS drive -- I don't see a large amount of data in Roaming.   That may be different if you installed in the default location.   Regardless, having data in AppData is not unexpected in this case, even if the particular choice of Roaming vs Local may be a little non-standard.