Tap the icon with three dots in the bottom-left corner and select the icon with a chest in the menu that will appear on the bottom of the screen. This is the list of all the Collections in the game. Here you can exchange the Collections and see the hints about place where to find a Collection and a reward for exchanging it. The rewards are listed in the top left corner of a Collection, the hints where to find a Collection are written under the list of Collection’s items.
You can look for collection items yourself or send your zombie workers (Treasure Hunters and Fishermen)