Ubuntu – Where are the syslinux x86-64 files located on a booted up live medium

The package-provided service files are all usually located in /lib/systemd/system. For example, search for .service in the package index.

From man systemd.unit:

/etc/systemd/system/*
/run/systemd/system/*
/lib/systemd/system/*
...

$XDG_CONFIG_HOME/systemd/user/*
$HOME/.config/systemd/user/*
/etc/systemd/user/*
$XDG_RUNTIME_DIR/systemd/user/*
/run/systemd/user/*
$XDG_DATA_HOME/systemd/user/*
$HOME/.local/share/systemd/user/*
/usr/lib/systemd/user/*

The latter ones are for user sessions. IIRC Ubuntu 16.04 still uses upstart for user sessions, so those files are only applicable from after 16.04.

For a specific service, to see what systemd is reading, run systemctl status <service> or systemctl show <service>:

$ systemctl show ssh.service | grep Path
FragmentPath=/lib/systemd/system/ssh.service
DropInPaths=/etc/systemd/system/ssh.service.d/override.conf
$ systemctl status ssh.service
● ssh.service - OpenBSD Secure Shell server
   Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enabled)
  Drop-In: /etc/systemd/system/ssh.service.d
           └─override.conf
   Active: active (running) since Thu 2017-01-26 16:06:53 JST; 21h ago
 Main PID: 948 (sshd)
   CGroup: /system.slice/ssh.service
           └─948 /usr/sbin/sshd -D

After a dialogue via comments the original poster, @Thoughtcraft, could work around the problem:

Installing the standard way (with the first option 'Install Ubuntu Server') works, and it is possible to install the HWE kernel later to get the desired Ubuntu Server system.