In one way, this chapter was a deep dive, but there is much more to learn about all the topics in this chapter. I strongly suggest that you read the man pages of all the used commands.
For storage, besides the documentation on the Azure website, some filesystems have their own websites:
- XFS: https://xfs.org
- BTRFS: https://btrfs.wiki.kernel.org
- ZFS: http://open-zfs.org
Lennart Poettering, one of the main developers of systemd, has a nice blog with lots of tips and background information: http://0pointer.net/blog. And on top of that, documentation is available at https://www.freedesktop.org/wiki/Software/systemd.
After this chapter, I strongly suggest you visit Chapter 11, Troubleshooting and Monitoring Your Workloads to read about logging in Linux, because often the systemctl status command doesn't provide you with enough information.