I haven't installed NetBSD, only OpenBSD, but i think the install process is the same as they have a common ancestor.
With OpenBSD, one boots into MacOS, partitions the drive (with a supplied utility), then installs BSD from MacOS. Only after the install is complete can you boot into BSD. All the installer program does is unpack .tar files onto the BSD partition. Those files can reside on a fileserver. Once BSD is installed, you boot into BSD and customize/configure it to suit your needs/requirements.
The way you actually boot into BSD is to first to boot into MacOS, then run a BSD boot application. One does not boot directly into BSD. (I think this is done to minimize hardware specific boot code.) This process can be made into a turnkey system because the booter application can be configured to automatically load BSD, and you can of course put an alias to it in the Startup Items folder in the MacOS System folder.