Stardew Valley stores data in .xnb files. XNB files are like zip files, in that they contain more files inside of them. Old mods edited xnb files directly, but this is no longer recommended thanks to the introduction of SMAPI, which allows users to add and remove mods easily and without risk of breaking their game.From the Discord server:
XNB mods often break the game and are not recommended. See https://stardewvalleywiki.com/Modding:Using_XNB_mods for more info (and a list of Content Patcher alternatives), and https://stardewvalleywiki.com/Modding:Player_Guide/Troubleshooting#Reset_your_content_files to fix problems caused by XNB mods.
For mod creators, see https://stardewvalleywiki.com/Modding:Editing_XNB_files for help unpacking & editing them (including for use with Content Patcher).