Hey, thanks for the cool words !

The effect is made of multiple sphere particles.

Inside a PostEffect Material we render the normals of each spheres and blur them together to get a smooth blend between them,

Then we apply a cubemap and a matcap on the spheres based on the blurred normal values.
We use a Stencil buffer to apply the Post Effect only to the particles.
The post effect is a bit heavy but I think the aspect is super cool.