Hi there, welcome. No worries about "dumb questions" or where they're asked, we're all here to help.
Indeed, most mods you'll find around are for the original game. You can only modify the Steam version with mods made specifically for it, and the two don't mix at all. I understand it may not look as deep as it is, but the two really are different games in such regards.
(The HD pack will also not change that; it restores visuals, but it does not change the fundamental differences of the two.)
In short, you can either look for Steam-specific mods to apply to the game you've purchased or you can acquire the original game to mod it with Revival and other such mods.
For the latter, you can likely still find copies for sale on sites like eBay and Amazon. I hear there are other, less savory means of acquiring the game as well, but of course we can't condone those.
