You can trigger on-demand workflows from inside a business process flow. For example, you can add an on-demand workflow to a business process flow so that an activity, such as a task or email, is created whenever a stage is completed.
Note: A workflow becomes activated based on where you drop the workflow onto the business process flow designer.
On-demand stage processes. When the workflow is dropped onto a business process flow stage, the workflow is triggered on entry or exit of the stage.
Reference:
https://docs.microsoft.com/en-us/power-automate/bpf-add-on-demand-workflow