That's the base game ~50GB, if you already have it you won't download it again. The optional mods are in the folder shown below. according to the date, i would download this, but i'm not sure myself actually.
View attachment 3535754
this mods contain maps, skins and much other stuff which can't be played in the game, it's for the studio-neo where you can create your own scenes, if you want to make e.g. your own VN with it or just for fun. The studio neo isn't easy to use for beginners, so be warned.