This is a continuation from here:
Crontab Info

the best/clearest info I have seen for using a cronjob with the preview generator app:
https://help.nextcloud.com/t/clarity-on-the-crontab-settings-for-the-preview-generator-app/6144/8

https://docs.nextcloud.com/server/11/admin_manual/configuration_server/background_jobs_configuration.html

$ sudo crontab -e -u www-data

then enter the code below at the bottom (runs every 8 hours):

* */8 * * * php -f /var/www/html/nextcloud/cron.php

How to run a cron manually (the example below is for nextcloud):

$ sudo -u www-data php -f /var/www/html/nextcloud/cron.php

Setup Cron (or systemd timers) for the Nextcloud Preview Generator

https://nextcloud.com/blog/setup-cron-or-systemd-timers-for-the-nextcloud-preview-generator/

Cron

Add or create a new cronjob for the Nextcloud user:

$ sudo crontab -e -u www-data

Using your favorite editor, add something like this, which will run the job at 04:00 (make sure to point to the correct directory of the ‘occ’ file):

0 4 * * * /usr/bin/php -f /var/www/html/nextcloud/occ preview:pre-generate

Or using systemd timers

Systemd also has the possibility to run specific tasks at specific times or events. called Timers. You need to create 2 unit files:

File 1:

$ sudo nano /etc/systemd/system/nextcloud-preview-generator.service

and add this to the File 1:

[Unit]
Description=Nextcloud Preview Generator

[Service]
Type=oneshot
User=www-data
ExecStart=/usr/bin/php -f /var/www/html/nextcloud/occ preview:pre-generate

[Install]
WantedBy=basic.target

File 2:

$ sudo nano /etc/systemd/system/nextcloud-preview-generator.timer

and add this to File 2:

[Unit]
Description=Run Nextcloud Preview Generator daily at 04:00

[Timer]
OnCalendar=*-*-* 4:00:00
Persistent=true

[Install]
WantedBy=timers.target

systemd reload:

$ systemctl daemon-reload

activate the timer:

$ systemctl enable nextcloud-preview-generator.timer

start the timer:

$ systemctl start nextcloud-preview-generator.timer