There are few games that deliver it's own Development Kit so you can decompile and modify the game assets in this case the geometry of the models by converting them to a general geometry file format that almost every 3D modeling software can use like Wavefront OBJ.
However, there are lots of games that don't deliver tools for unpacking the assets and some games even restrict it by encrypting the packages that contain the assets with proprietary encryption tools which you can't find anywhere on the internet to decrypt the packages.
My idea: Every 3D Game, no matter what engine it runs on and how well the asset encryption is, have one thing in common: they use geometry, meshes that has to be rendered by the graphics card.
Is there a way you can get in between the game and the graphics card to take the geometry that has been sent by the game to the graphics card and convert it into a obj file so you can use it in blender?