It will vary from designer, sometimes you can find clipart or a background that can be used but if there is nothing out there that is large enough to use I just draw my own.
I use paint shop pro, I goggled tutorials to find ways to do different things, and mostly it is trial and error. I use lines and shapes to draw most things and gradients to shade, somethings I draw by hand. Here is my most recent creation. The only thing that is clip art on this one is the necklace. The rest was created by me with shapes and drawing, a new mickey head like this can take me 3-5 hours as I tend to redo things as I go along:
View attachment 231596