How to create a background in Adobe Illustrator?

Unlike Photoshop, many designers/illustrators don’t start by flooding the artboard with color.  You have many options.

• You can create a rectangle shape and fill it with color
• create gradient fill
• you can place an image
• place linked project

Basic Rule: Your bottom layer will be your background, barring advanced exceptions.  No bottom layer? Then your artboard dictates background.

(Illustrator Draw (mobile app): To create a flood = draw a closed shape larger than your artboard. (You might need to zoom out a little on your mobile device to see the edges).  Long press in the center of your closed shape to flood it with color.  Optionally, you can change the color to anything else later by using a similar long press on that layer)

a flat color background could be a rectangle with a filled color. you have to give us more information so we can help you. Otherwise we are all just guessing here.

a background for what?