People generally find their way there via this wiki, which covers what you need to know:
The expectation for the docs in plugin repos are that you already understand how to use Caddy and have read the docs. It can’t teach you everything, because then information would be repeated and more easily made outdated over time. Bitrot etc.
That’s because you are “off the reservation” as you put it. libdns plugins are community maintained.
Right, because it would be redundant to repeat that information when it’s best described by the official AWS docs.
That’s fair, we could better document that the $HOME for the systemd user is /var/lib/caddy. Assuming you went through Install — Caddy Documentation then you could have clicked the caddy.service link and seen in the service file User=caddy which should lead you to find that user’s home (tip: run eval echo ~<the user you want>, e.g. eval echo ~caddy which will output /var/lib/caddy)