Just plugging in the hard drive will mount it and allow for you to store files on it like a regular flash drive, and you can use it for multimedia, storing your video, music, and photos on it, then viewing them with Kodi, or your multimeda app of choice. Basically anything that isn't an app can be stored on it and loaded from it with the corresponding app. Fopr example, I use an app called Joiplay to play RPG Maker and Ren'py games, as well as Retroarch for emulators. I store all my game files on a regular flash drive.
You will not be able to use it as extra storage space for intalling apps and app data however, without the use of an app or Xposed/Magisk module.
Personally, I use Apps2sd Pro to "link" (or move, to put it simply) apps to a 20gb EXT4 partition on my 32gb microSD card that is in my Fire TV 2, it's the easiest method that I was able to figure out how to use, but there are other ways.