Downloading docker image instead of puliing it [closed]

  containers, docker, nginx, php, symfony

I am a beginner with the docker. I knew some basic commands. I dont understand how that man pasted the docker image link. Did he clone some github repository and then he put it into symfony? Next step was that he also pasted docker-compose.yml out of nowhere so I have no clue how to do it on my own. Can somebody explain me it to me, please?
P.S. I know about existence of hub.docker

Source: Symfony Questions