I don’t know if it has been mentioned in the notebooks books and I missed it…
There is a good cheat sheet with ROS2 commands called ROS2-Command-Cheat-Sheets-updated.pdf .
I have found it very helpful to download it and leave it open in the corner of my second monitor while I am studying ROS2. It is great for figuring out what command will work to help me figure out things like “What is the interface type for an empty service interface?”