Some will be made with crayons, pencils, markers, etc.
And then, digitazing them to play with them inside Photoshop
 While others will be done using only Photoshop to draw them.
A beautiful Easter image from a royalty-free clip art art (below).
As well as examples of doodles that I have drawn:
This site is for anyone interested in doodling.
Udemy.com has great cartoon classes.  i enjoyed drawing these facial expressions
When ready, it will give links 
to my favorite tutorials on how to draw &color on paper 
or digitally.
Try replicating simple forms

These are simple enough to be done with a pencil or thin marker on a piece of paper

 or using a 
digital tablet.