The Setup App when added to a step runs all its child steps once when the end-user sets up the integration. It comes in handy when a Flow has a Query that needs to be run once at the time of integration creation. Setup app can only be added on the root level of a Flow.
Let's see how to add the Setup App step to your Flow.

Step 1: Search and add Setup App from the App Connector selection menu.

The Setup app has no configurations and can be used as it is. Click on Done to add further child steps to your Flow.

Step 2: Next, search and select the app you want to add to the Query step.
You can add as many child steps to your Flow after the Setup App step as you want.