Oh my god. Nobody didn't read "READ ME!" ? I wrote what possible to copy "default" folder and edit a copy, then change copy name by your own, and you can drop other assetpacks in "MSITUA/Assetpacks" and launch them by writing it's name in text box in assetpack load window (black windows that summons by pressing "folder" button in main menu).
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and also I just figure it out why people decided to do that way. Well because for the youtuber. Like some youtuber didn't know how to launch the mod so that why they replace the asset in the "default" folder so they can play it right away. Again sorry if I did something wrong when trying to mod the game.