School #1/ original illustration
I will roughly explain the process of creating illustrations.
The application I always use is Adobe Illustrator.
The image for texture uses what I made or free material.
1) Trace the bottom drawing.
2) Then color it.
3) Divide the object by color.
4) Add a texture while looking at the balance of the whole.
(I use opacity mask here)
5) When using illustration for offset printing, pay particular attention to the color density.