I wasn’t aware that systemd is a dependency of snapd. This should perhaps be stressed in the documentation.
I can follow your advice and use the process-control interface to find if systemd-resolvd and/or systemd-networkd are running.
I read Enable/Disable systemd services and had a look at the dbus interface: it looks like snaps cannot initiate communication with the host system via systemd or D-BUS. I’m still trying to understand how I can get through confinement. I guess I have to read through all interfaces, but that’s clearly not enough. Understanding the details of confinment and how to interact with the host system is probably the toughest part of the learning curve.