Explanation:
Box 1: Azure Blob Storage
The source of the event, which means WHO has created the event and who is publishing this event is the Blob Storage.
Box 2: Azure Event Grid
Event Grid uses event subscriptions to route event messages to subscribers. This image illustrates the relationship between event publishers, event subscriptions, and event handlers.

Box 3: Azure Logic App
Handler of this event, which means who should handle this event, is the Logic App by creating a subscription in the event grid and connecting the Logic App to that subscription.
Reference:
https://docs.microsoft.com/en-us/azure/storage/blobs/storage-blob-event-overview
https://docs.microsoft.com/en-us/java/api/overview/azure/messaging-eventgrid-readme