So I am an old-timer who was happy with SysV init and have never really managed to find documentation which completely details how systemd works in a way that makes sense. It seems like your article series is going to help me with this. I read as far as the systemd configuration link is /etc/systemd/system/default and my hopes are dashed. My system definitely runs systemd and it has no such file. What distribution/version is this series covering? I'm guessing it is some version of Fedora/RHEL, but can't be sure. In case you are wondering, the distribution/version that I am running still gets security update so it's not like it is that old. Do many distributions fundamentally change things like this or has something so core
to systemd actually changed in the last 2 or so years? In any case, you should prominently display on every page of this series what version of systemd you are actually documenting so people who get here via google know that this isn't for them.
One other thing, your description of the boot process left out one potentially major step: loading/running the initial ramdisk. I have to admit that I haven't looked at one in a while, but my system still uses one and it has gotten quite complicated. No reason to do more than mention it in this context, but it would have been nice to do so.