I'm talking about when you install the game, there is nothing in the meshes/textures folder. When people make mods, however, the replacements are put in those folders and they somehow overwrite the original meshes/textures that I can't find. How can I find those originals?

They are packaged in the BSA. (Bethesda Softworks Archive)
Mod files overwrite the ones in the BSA because they have a more recent timestamp.

You need a BSA tool to look through the BSA and extract files.
http://mw.modhistory.com/download-95-5496 is what I use, but there are others too... http://mw.modhistory.com/download-95-12389

Also, if you have a hard copy of the game, you can pull files from the Construction Set disk itself.

(this is also why some some mods come with no loose files, like TR, because they use a BSA instead)

Cool, thanks a lot!