The systemd website itself seems quite information-rich: https://systemd.io/

read the man pages. type "man systemd" into a linux terminal, and when finished also read the "see also" pages at the bottom. man systemd.unit is also a "central" page, it says lots of things common to all unit file types.

when you stumble into long parameter lists, you can skip them, you probably won't use most of them. not because they are useless, though, so it's better to at least read the names of all the parameters you come across that way so you have a picture what's available.

skip systemd.directives, but know what it is: a catalogue of all systemd directives with the man page they are documented at. very useful, when you want to find something specific.

"man systemd.special" is special, it's more about its internals, very informational, but relies on preexisting knowledge

I'm not experienced at it either and don't know the best resources.

But what I can usually recommend in case you don't want to see the usual "THIS-IS-A-PIECE-OF-THE-PUZZLE---COME-BACK-REGULARLY-FOR-MORE-CONENT" stuff, but more in depth stuff: Enter "filetype:pdf systemd" in your search engine. Google or DuckDuckGo will then only spit out pdf files about that topic... And the people who write PDF files are usually more experienced with the topic than those who write blog posts or "how to's".

Let me know if that helped in your case... :)

Diving into Systemd would be a book written by Nietzsche.