I used the itch.io app a lot, but because of where it drains my GB (C:) , I rather start downloaded using the browser and itch.io webstite.
Now I don't really understand how people add dlc or updates on "normal downloads". Maybe I am getting confused by something because I might be doing it right already, but I just got a bit worried because there was a time, where I had to add something in another folder and stuff to update a fix or something.