Thanks a bunch @sven2 , @Manou, @BG_Division and @julperado !
Here are more insight on the rig ! The core is pretty classic, like arms/legs
uses IK/FK with additional features like stretch and bend, thanks to blender’s b-bones.
Since I’ve done a lot of rigging in the past I came to make my own auto-rig based on modules. Stuff like arm, legs, car wheels… can arbitrary be put together very similarly to rigify.
It also supports things like shape keys and lattices, and I can improve it from project to projects depending on my needs.
On this character some part are very similar to the Boon and Pimento rig demonstrated in this video/post.
Also, the main parts are pretty simple, which makes rigging easier and also it allows to create other characters with the same style quickly…
The most advanced part is the mouth, which at the core isn’t that complicated either…
I uses a curve as a basis that is shaped into lips and a boolean cutter for the inner parts :
The curve is then rigged in a way that recreates the handles of the curve but with bones :
I take a lot of inspiration from that addon to incorporate the same functionality to my autorig system :
So, relatively easy stuff on the overall but we all know that the devil lies in the details …
For instance to get rig control over the radius of the curve I had to make some hacks using geometry nodes :
The trick was to use a mesh where each point represents curve’s radius, those points are moved by bones and they get sampled with geometry nodes. So basically the point’s height represent the radius of each points of the curve…
Also I wanted the inner parts of the mouth to scale with the curve, and this turned out to be some kind of a rabbit hole…
For that I used an intermediary mesh strips that is deformed along the curve.
And to make it stretches as the curve extend I had to compare the curve’s initial length and the deformed one, I even made fancy visualization tools while debugging the whole system.
Eventually I transfer the deformation of the plane to the teeth… Then I can set some influence for that or add an extra bone transform to tweak further the teeth…
From there things slowly get back to normal, the eyebrows uses the same curve rig system :
And the eyes have those 2D lattices that allows different deformations…
And that’s all I can think of right away…
Well, I moved on to something else for now, but I’d like to explore more those kind of cartoony techniques at some point, so I suppose it will come back in one way or another.
Parts of my todo includes using those characters as an opportunity to study animation but that todo is a bit dense already 
Thanks everyone again !