If it dynamically creates systemd units or sets their properties (systemd-homed, systemd-networkd, systemd-resolved) using the dbus endpoints, it's systemd-something.
If it uses systemd libraries and lives in the systemd repo because it shares baseline docde, it's systemd-something.
systemd is modular in the sense that those components are build-time flags. For homed, for example: https://github.com/systemd/systemd/blob/main/meson_options.t...
Again, not confusing. Literally looking at the systemd repo for longer than 30 seconds would show you both why this it's considered modular, and why it's systemd-whatever
Pulse doesn't.