i would say the look comes mostly from the “hand painted” textures.
partially there are a lot of broad, rectangular brushes used. look at the sky or the road.
generally the brush strokes are loose and visible. they have a “painterly” style.
the lightning is also playing a big role. it´s a rather non realistic style. i guess its a mixture of flat shading (mostly the backgournd) and the use of a fall off shader which creates a “soft” look (especially for the characters)
another thing is the choice of color values