In order to build a Google Assistant app, Google provides two different APIs as part of their node.js actions-on-google library :
IN SIMPLE TERMS
Use the Actions SDK for one-shot apps. These are apps that provide the required answer directly after being invoked and then stop. They give you just this one result. A typical example would be setting a timer to ten minutes.
Use the Dialogflow for all other apps, for those that are really conversational, where there are multiple paths to follow and where you want your user to provide more information during the conversation.