Most of the mods actually just modify the game's xnb files, meaning you download a modified xnb and replace the corresponding one in the game's 'content' folder.
Stardew Valley Mods Folder / When using stardew valley mods, you will be able to make some changes including the game mechanics, for instance, making the fences to decay at a slower rate, you can also make some cosmetic changes like changing the appearance of your house among other things.
