When Google Chat sends an event to an app, it includes certain data in the event payload; the exact payload depends on the event type. This event and payload are contained in the HTTP call sent to the app's registered URL.
For some events, the app might send a message to the chat in response to the event. This message is a JSON object, whose contents depend on what kind of message it is:
These messages are displayed like any other chat message. They can include simple character formatting.
These messages contain all the details that Google Chat needs to render a card in the Chat space. These details include any text, special formatting, widgets, and other details used in the card.