101-tutorial | This tutorial has been written with the intent | Continuous Deployment library

I am new to Docker and am trying to learn about Docker using 101-tutorial docker image(https://github.com/dockersamples/101-tutorial)

In this lesson, they were trying to explain that when 2 container instances are run from the same image then the data created by the first container is not available in the 2nd container by default (unless we are using container volumes and mounting, etc).

1st and 2nd points shows that an ubuntu container is running and has a file called data.txt.

As per my understanding, docker -it is used for getting access to terminal inside a docker container. https://docs.docker.com/engine/reference/run/

But I don't understand the command used in 3rd point. Why is docker run -it ubuntu ls / command run on Ubuntu image rather than on a container using Container ID? Is the command used for just showing the contents inside an image instead of showing contents inside a docker container using ls /?

I expected that 3rd point should have created another container instance and then run ls / but as per docker ps I see that there is only one ubuntu container running in my machine.