Illustrations are composed by several elements and plans.
Structure & plans
Avoid to use 3D elements in illustrations. To create a perspective effect and create depth, prefer to play with elements size and colors. Thereby, use shade of grey for background elements.
Added elements & details
To create dynamism, don't hesitate to use these little elements in your illustrations. According to the illustration size, adjust details. More the element is small, the fewer details there are.
Using a background helps to highlight an illustration. Try to use circular backgrounds as much as you can.
ffffff , F4F4F5 , and for ibbü and emails CCEEFF
To create depth in illustrations, the background can be smaller than the content. Avoid a too big size difference between background and content.
The most of illustrations are based on geometrical shapes. Don't hesitate to add curves for people illustrations.