how does isc-dhcp-server start on debian 10 with systemd

The packaged services normally have their .service files at /lib/systemd/system/.

But the package maintainer may have been relying on the SysVinit compatibility mechanism: if there is no .service file at all for it, but /etc/init.d/isc-dhcp-server script exists, systemd will auto-generate a service definition for it, setting the service dependencies based on the LSB structured comment lines in the beginning of the script.

You can always use systemctl cat isc-dhcp-server to view the current service definition, and systemctl edit isc-dhcp-server to create a persistent override file to make changes or additions to the service definition - even if the main definition is generated dynamically from a SysVinit script.