ED32 and ED64 are the base game and required
EDH is the DLC
Yes and no.
Elite Dangerous: Horizons (64-bit),
Elite: Dangerous (32-bit) and
Elite: Dangerous (64-bit) are separate SKUs and used to be installed into separate directories. OP is right that Steam used to want to download all three, which was a waste of time, bandwidth and disk space if you only wanted to play Horizons.
Nowadays there is no 32-bit client and I believe that the base game and Horizons install into the same directory (don't quote me on that) but it wouldn't surprise me if Steam wanted to install everything from a clean start.
You can bypass Steam and run the Frontier launcher standalone, which will let you choose to install only Horizons. Note that if the game is left "installed" on Steam it will still want to auto-update. That can be useful if you leave Steam running all day, as when you get home from work you will find it has downloaded patches for you in the background.
It's possible that if you run the launcher and tell it to install into the same location that Steam would, ie
\path\to\steam\steamapps\common\elite dangerous, that Steam will figure out that it doesn't need to redownload everything.
Having said that I have, after recently upgrading my OS, over 350GB of fully and correctly installed games on a separate drive which Steam pointlessly wants to download completely from scratch, so I have been reluctant to let it anywhere near validating files on anything ever since...
As an aside if you run the launcher with the
/edh flag it will only offer you Horizons (and any currently active betas). When the game is up-to-date all you see is "Play" without the list of other SKUs you aren't interested in.
