Actually, BG and Fallout have nothing in common in the way they were created. BG and the Infinity Engine games in general have more in common with PoE than with Fallout and Arcanum, or even Diablo 1 and 2.
Infinity Engine games, PoE, Torment: Numenera etc all have pre-rendered or hand-painted backgrounds, usually a combination of 3D render with some painted touch-ups. This means that every map needs to be created by artists, and when the final version is done, it's difficult to make changes to the map since they have to import it back to the 3D editing software and modify the map there, or put it in photoshop and have artist paint new stuff on it, etc.
Meanwhile, games like Fallout, Arcanum, Diablo etc use a much more convenient way of level design. The Shadowrun Returns games, too, btw, which is why they could make 3 games in a row in relatively quick succession.
In those games, they build the maps with a levele editor, where the maps are built from tiles which can be textured individually, objects are assets that can be freely placed in the level, etc. Just play around with the Shadowrun Returns level editor to see how it's done.