How to change the background of an object?

Question

Adobe Illustrator

1. How to change the background to the balls on the right instead of a solid fill of the ball?
2. How to add these balls on top of the main color? Put a duplicate ball on top?

Answer 1

1. Learn the Pattern function - this is from native. There are also scripts and plug-ins that allow you to fill one form with any other, and even with the rotation, transformation and scaling of each element of the fill...
2. Yes

Answer 2

Easier (but more crutch) You make
balls in many rows and columns and a mask (your circle)