What can I do with the default collection "Device" in Apigee BaaS?
The docu says http://apigee.com/docs/app-services/content/default-data-entities The device entity represents a unique device that is being used to access your app. Device entities should be associated with a user entity. The API Services push notification feature requires the device entity.
How do I really fill the Device collection?
The most convenient way to fill this collection is by using one of the Apigee BaaS SDKs. When installed on a mobile device, you can trigger a device registration which will include the unique device ID and a push token.
You can read more about it in the Apigee docs.