Any examples of library-in-app in the wild. I get the idea, but want to know what this looks like in practice. Mounting engines or Sinatra apps? Creating a new

Any examples of library-in-app in the wild. I get the idea, but want to know what this looks like in practice. Mounting engines or Sinatra apps? Creating a new gem to for extractable functionality? Just curious about what this looks like under the hood.