Here is a visual on how the ports are mapped in the docker-compose file. You can see from the image, if you, the user, use your browser and navigate to
http://dockerhostipaddress:3001 you will be routed to docker host's mapped port for Tautulli. From there docker translates that port and redirects you to the docker container's (Tautulli's) internal port. That is where the container (Tautilli) is publishing the website that would get fed back to you.