Using artboards in Illustrator is very much like pages in programs like InDesign.
When you create a new document in Ai, you can choose to create multiple artboards as well. You can also add artboards once in the document.
To do this, begin by selecting the Artboard tool. Then, you can draw on the canvas to create a new artboard and resize using corner handles
In the Essentials Classic panel, you can click the plus and add an artboard of any size from the top control bar. In Essentials, this is in the Properties panel. You can resize artboards by choosing various presets in these locations.
You can also duplicate an existing artboard by pressing Option (MacOS) or Alt (Windows) and drag an exact copy of the artboard and its contents. This is great when you a creating different mockups for clients. You can rearrange art boards by dragging them.
Finally utilize the artboard panel to name artboards and to rearrange them.