I have ros-galactic-base installed on my Raspberry Pi4 2GB robot, and have ros-galactic-desktop installed on my desktop computer (running over Ubuntu 20.04 Desktop in a virtual machine on a MacMini 2018.)
The Raspberry Pi 4 is not where you want to run ROS2 visualization tools (rviz2, rqt) or the simulation tools (Gazebo), but I am able to run my robot nodes, image publishing node, and the mapping slam-toolbox node using 600-800MB of memory and 25-35% of the Raspberry Pi4 (2GB in my robot's case).
For learning ROS2 I suggest you stick with only installing ROS2 Galactic ros-galactic-desktop over Ubuntu 20.04 Desktop on your desktop, and leave the Raspberry Pi for later investigations of distributed ROS2.
You will use the turtle sim tool at first on your desktop to do the initial ROS2 tutorials, so you don't need a robot either.
The reason, at this time, I am suggesting Galactic on Ubuntu 20.04 versus Humble on Ubuntu 22.04 is that some of my robot nodes have issues running on Humble that do not occur on Galactic.
My second question is: Could I install ROS 2 on the Ubuntu 22.04.1 LTS? Is It must be 20.04 LTS?
The short answer is yes, the more nuanced suggestion is use what the particular ROS2 release says it was tested on - Galactic on 20.04, Humble on 22.04, and mind my feeling you are better using Galactic at this time.
Good for you investigating ROS 2 - It is a HUGE domain to learn in, so have patience with yourself, and really search hard for answers - many questions have been asked before, but they will all be new to you.
