Ubuntu OS (Operating System) is a distribution of Linux OS and is a derivative of Debian OS which is another distribution of Linux OS
A lot of people use Ubuntu for their Linux Servers and even as their Desktop machine OS. I personally use Ubuntu a lot when working with Linux VMs, for playing around with things. It's free, and it has the most basic things. It's also the one distribution that I have used a lot of. While many hard core linux nerds might talk about other light weight and fast Linux distributions and fancy ones too, like Arch Linux etc, I personally like Ubuntu
In Ubuntu, a popular way to install software is by using apt (previously apt-get). apt is a package manager for Debian and it's derivatives like Ubuntu. apt is short for Advanced Package Tool (APT)
apt has CLI (Command Line Interface) interface - basically it's a command line tool, not some fancy GUI (Graphical User Interface) program with windows, buttons etc
When you do apt install -y python3, it installs Python v3.x.y for you
