The artwork was drawn roughly by hand with a pencil at first to sketch out the idea. After scanning in the illustration, many touch-ups were made in Clip Studio, including roughly drawing the same illustration again digitally.
The final result of this drawing features clean line art, as well as color with shading detail that mostly uses a digital watercolor brush.
Below is a rough idea of the creation process.
You may view this illustration on Pixiv as well.