systemd is the PID 1 init system on RHEL. It manages units — services, sockets, mounts, timers, targets — and decides what runs at boot, in what order, and how failures are handled. systemctl is the single CLI you use to talk to it: start, stop, enable at boot, mask, view status, read the unit file. On the exam, "make it run now AND survive a reboot" is the pattern you'll see over and over — that's always systemctl enable --now <unit>.
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