Here's a concrete example of using dynamic content in Power Automate to update data on a Sharepoint List from PowerApps:
Let's say you have a Sharepoint list named Inventory with columns ProductID, ProductName and Quantity. You want to create a workflow in Power Automate to update the number of products in this list from PowerApps.
In PowerApps, you created an app with an input field for ProductID and an input field for the number of products to add. You also added a Submit button to send data to Power Automate.
In Power Automate, you create a new workflow with PowerApps enabled and add an Update item step to the Sharepoint Inventory list. In this step, you select the ProductID field and use the PowerApps trigger dynamic content to select the ProductID value imported from PowerApps. Similarly, you select the Quantity field and use dynamic content to select the product quantity value imported from PowerApps.
When the user enters data into the PowerApps application and presses the Submit button, the data is sent to Power Automate and the workflow is activated. In the Update item step, the ProductID value and product quantity will be retrieved from PowerApps via dynamic content and updated on the Sharepoint Inventory list.