I'd suggest that the principal udeb package to consider as responsible
for installing the base system is bootstrap-base. It runs debootstrap.
Debian Policy specifies the base packages as being of Priority: required
and Priority: important.
You can get lists of these packages with
aptitude search ~prequired -F"%p"
aptitude search ~pimportant -F"%p"
Any idea which of those installs systemd, and if that can be
modified through a preseed command?
The init package is Priority: required. It pre-depends on
systemd-sysv | sysvinit-core | upstart
The first alternative is systemd-sysv, which pre-depends on systemd.
There is no preseed command which can alter this.