Generally there are two ways to load a mod - from a .pak or from a set of folders

.paks for mods go in
C:\Users\***YOUR USERNAME***\Documents\Larian Studios\Divinity Original Sin\Mods\

set of folders go in
SteamLibrary\SteamApps\common\Divinity - Original Sin\Data\MODS
and
SteamLibrary\SteamApps\common\Divinity - Original Sin\Data\Public


The main .pak is in
D:\SteamLibrary\SteamApps\common\Divinity - Original Sin\Data
but i'd suggest doing anything you do as a mod.

If you just start sticking .paks in and around places in your DATA/MODS folder, it wont work, the game doesn't look there.