Krysidian used Edward Ureña's Detached Facial Feature.
Check out this cute little girl and her different moods in Krysidian animation that looks like it was hand-painted. The artist used Edward Ureña's Detached Facial Feature approach to diversify the girl's emotions, and it's obvious that Ureña's rig suits not only Pixar-style images but also other art, including one made with Blender's Grease Pencil.
The modular rig has a simple head and a mouth powered by Geometry Nodes. You can see how it works in this video:
Krysidian has worked on mouth rigs before this setup, including a tool to draw a 2D-like mouth on meshes. You can find these experiments on X/Twitter.
If you're also impressed by Ureña's work, take a look at his Elemental-inspired tool based on curvenet animation controls and this cool rig of what looks like Jack Skellington from The Nightmare Before Christmas.