In Archlinux, crontab is not installed by default because of systemd. Doing system upgrade every day can be achieved by writing timer unit and oneshot service of systemd.

By default, user-defined systemd unit files (.service, .target, .timer…etc) is located in /etc/systemd/system

1. service file

   see man systemd.service for more info.

        # /etc/systemd/system/system-upgrade.service
        [Unit]
        Description=Full System Upgrade
   
        [Service]
        Type=oneshot
        ExecStart=/usr/bin/pacman -Syu --noconfirm
   
        [Install]
        WantedBy=multi-user.target
   

2. timer file

   see man systemd.timer and systemd.time for more info.

        # /etc/systemd/system/system-upgrade.service
        [Timer]
        OnCalendar=daily
        Unit=/etc/systemd/system/system-upgrade.service
        [Install]
        WantedBy=multi-user.target
   

3. enable the timer

   sudo systemctl start system-upgrade.timer & sudo systemctl enable system-upgrade.timer.

4. reference systemd/Timers - Archwiki