If you are talking about it like this

it just works like any other subfolder inside of the app folder. You could add a page.tsx/jsx file to create a route for example
domain.com/app

the [domain] folder i believe is a dynamic route which would be accessed by domain.com/[domain] and you replace [domain] with a parameter you'd like to pass like domain.com/himay then under page.tsx/jsx of the [domain] folder you can pull the himay parameter and do what you'd like with it