At the top of any design category you click into in the Brand Kit (e.g. Facebook covers), you'll see two tabs: Made for you and Saved designs.
Made for you will be all your generated designs. When you click any of the designs in that category (or edit them or download them), they're going to automatically appear in the Saved designs tab.
Think of Saved designs as keeping track of anything you've showed interest in or worked on.
Note: Saved designs appear in a low-res format for fast loading, so they'll look blurry on the Saved designs tab. However, when you click into the editor or download them, they'll be high-resolution files.
You can also check out part of this video that covers how you find saved designs in your Brand Kit (starts at 1:32).
Need more help? We have a Brand Kit webinar that goes into more detail on how to create awesome marketing materials for your business.