Blender Made Easy covers everything you wanted to know about instancing collections in Blender 2.9.

Instancing is a familiar friend in the 3D world. If you work with Blender 2.9, you can instance collections much like you can with other instance workflows.

This new tutorial from the Blender Made Easy channel shows how you can use instancing collections to unique designs pretty quickly. The technique also allows you to animate the visibility of collections within Blender.