Yes? People don't realize that systemd is a project, and systemd the init system isn't journald or systemd-boot or systemd-resolved. Those are other compatible utilities by the same project. They aren't required to use systemd itself.

Many of the systemd components are separate and optional. You can easily eschew from systemd-timesyncd and systemd-resolved, for example.

Not entirely, for example:

Units can depend on networking managed outside of systemd-networkd using the target -- think of this as a 'run level'

Units can depend on mounts outside of systemd .mount units -- there's a generator that considers fstab

At a certain point, you might want to go further into systemd... but it doesn't require it.

The basic 'this service wants/needs these other ones' is very self-contained, and inherits a lot.

Yes. It needs to know what mounts and networks are needed for my various services. It needs to be sure the system is synced with NTP before moving past a certain point. And so on.