where is the apt update looking at?

  docker, ubuntu, ubuntu-20.04

I installed docker on my Ubuntu desktop (20.04.3 LTS) and then uninstalled it.
So when I run

sudo docker

it says command not found.
However when I run the following command,

sudo apt update

It gives me the following,

enter image description here

I am very new to Ubuntu and I assume there must be a file that it is reading to get workout which packages it needs to update.
So how do I clear the docker messages?

Thanks in advance

Source: Docker Questions