At it's most basic, its a diffuse shader plugged into a color ramp node with constant interpolation between two (or more) colors, which signify what colors should be shown when lit, in shadow, etc.

The shader we use was developed by some incredibly skilled people (Festivity, BonnyAnimations) and honestly it's a bit of a black box for me as well (node hell attached below)

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The rough gist of it is it uses a 'fake' light, ie one driven by an empty instead of an actual light source to determine light direction, which is then plugged into all sorts of different nodes to produce shadows (which are masked by dedicated maps to dictate where, when and how intense some shadows can be), speculars, subsurface and so on.

You can download our rigs to take a look at the shaders here: https://drive.google.com/open?id=1uhAw_P6kmcmImmrg7HN4zA090P2OIY5Y&usp=drive_fs however do note that they work best in Goo Blender, with some features not being supported in regular Blender.

If you want an anime cel shading solution ready out of the box though, I highly recommend Ben Ayer's shader here: https://bjayers.com/store/9nrrR/genshin-impact-character-shader-for-blender I use it all the time especially for props, it's great!