Multiple syncs is legacy. Modern Linux handles it automatically. There is no real difference between reboot, shutdown, halt, and poweroff when using systemd.
The init system (using systemd) catches the halt command and begins a graceful shutdown sequence. It is the init system that sends signals, such as SIGTERM and later SIGKILL, to other running processes to allow them to shut down cleanly. Applications can use sigaction to catch these SIGTERM signals and perform cleanup tasks.
Under sysvinit, the halt command typically invokes shutdown to perform a controlled and graceful shutdown.