The tftp boot daemon on the apt server has its configuration file in the file /etc/default/tftpd-hpa.
The config file points to the filesystem location of the deliverables and a few options.

The deliverables we're using in the netboot setup are:
grub (a directory)
initrd.gz (the primordial OS that gets booted)
linux (the kernel)

The grub directory contains:
grub.cfg (the config file that provides the menuentries you see on-screen among other things)
grubnetx64.efi.signed (the boot app that actually boots the machine - AKA bootx64.efi)
theme (a directory containing pngs and the theme.txt file that arranges the pngs on the screen)
unicode.pf2 font (I think this is fall back font that can be removed, but I haven't tested the theory).

If you need to start/stop/restart the tftp daemon:
sudo systemctl start tftpd-hpa
sudo systemctl stop tftpd-hpa
sudo systemctl restart tftpd-hpa

To watch it at work, do:
sudo tail -f /var/log/syslog | grep tftpd