Installation folder for the mods needs to go "Mods":

"SteamLibrary\steamapps\common\TEKKEN 8\Polaris\Content\Paks\Mods\"

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Installation folder for the mods needs to go "~mods":

"SteamLibrary\steamapps\common\TEKKEN 8\Polaris\Content\Paks\~mods\"

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Tekken 8 Boss Trainer can go anywhere desired as it is an ".exe" trainer rather than a mod.