This is the brush that I use for all of my sketches and most of my shading, I used it for shading and sketching the thumbnail, as well ^-^
Here's an example of a sketch I did using this brush and erasing sometimes with the transparent colour:
Then, I did the lineart with a harder brush, like GPen and filled in the colours with a the 'Enclose & Fill tool'.
After that, I shaded with this brush and added a few deatils using Clip Studio's Default brushes, like the clouds, stars and a bit of hatching for a prettier effect.