The application pool shares the web applications on the server, so it behaves completely independently of other web applications, so if another web application does not work, it will not be connected with its separate. The workflow basically handles all requests for the web application. I think that usually one application pool uses one workflow by default, but it can use more. In the process explorer, it will appear as W3wp.exe. As for the difference between the application area and the application pool, I am not 100% sure, but the answer has already been given on this forum:
Difference between application domain and application pool?