The artist used a combination of Geometry and Shader nodes to accomplish the task.
Digital Artist Gaonirico, known for creating stunning stylized shaders, has demonstrated yet another cool project made in Blender. This time, the artist has shown fantastic procedural clouds that can be easily recreated by Environment Artists who use Blender in their workflows. According to Gaonirico, the project was made using the combination of the software's Geometry Nodes and Shader Nodes. Here are the node graphs shared by the author that should help you if you are willing to create similar clouds:
And here are some of the artist's previous projects: