With raster paintings it’s easier just to pick up the brush, set a color, maybe a brush type (eg, airbrush, pencil, watercolor, etc) and just paint away. Before you start painting you MUST define the size of your illustration and need to think about whether you want to print it or not, and if so, then what size. You cannot scale UP your painting afterward without degrading the image quality.
Vector illustrations are much more elaborate (and harder to learn to be proficient) and you cannot just leisurely just paint away, as in PS for example. However, you can scale up or down your illustration to just about any size you want without any degradation to the quality of your work.
You can browse some very fine vector illustrations on this site:
https://www.illustratorworld.com/
You can see excellent technical illustrations here with a combination of AI and PS:
https://www.khulsey.com/
For some artwork (like the cutaway cars), he creates the outlines in Illustrator and colors them in PS.