I haven’t been in the weeds with systemd for a while, but this post was very helpful in understanding how systemd actually works, which didn’t match my (or, I would think, nearly anyone’s) intuition as a user of it. I don’t know if there’s any other full technical explanation of how units, jobs, transactions, etc. are actually handled.

https://blog.darknedgy.net/technology/2020/05/02/0/index.htm...

systemd’s developers and almost all public documentation revolve around a “unit-centric” definition of systemd (except when discussing bugs), but my position is that for systemd-the-service-manager the most important construct is not the unit but the job, and hence one ought to understand systemd in a “job-centric” way.