Some textures are from photo textures section of "textures.com" (I had some, before they went fully paid), some were kitbashed from photos found in google, for example car texture were stitched together from many photos of Peugeot 405.
But basically, just low resolution textures from real photos, i've personally used density of 64x64 pixels per meter, for consistency. Also, some slight posterization, to limit image palette to simulate 16 bit colors helps too.