It is according to the wishes of everyone, but you can absolutely buy DLCs via ORBX, and it will automatically create a subfolder in Steam, and this does not interfere with the use, except that you must first of all install FTX Central 3.
Concerning Aerosoft he will create himself a folder in documents. In the end, it's personal and everyone manages their path!
....it's personal and eveyone manages their path!... thats a very fine conclusion on what to do with files that are there posted!