the current build assumes the Larian Mod manager is used
1 if you have the current build it should include their manager or it can be downloaded from Gog as a stand alone but will only install as one or the other
2 the actual mods are mostly small so using the manager is generally the best option however some mods are also downloadable without the manager... its a case by case senario
3 assuming you downloaded an officially supported mod then it needs to be unpacked and tell the mod manager where that folder is... people report mixed results and it tends to work fine if you only use one mod at a time or use all mods installed the same way but breaks when you try to mix them
i have personally tested this on my Mac but not Windows... i let the manager do its thing there and so far so good