To fetch the latest Raspberry Pi OS package list:
systemctl start fetch-repo-package-list@$(systemd-escape https://archive.raspberrypi.org/debian).service
The service makes use of systemd's CacheDirectory, and RuntimeDirectory during execution. The updated package list is outputted to the RuntimeDirectory, which would typically as follows for the above example:
/var/run/fetch-repo-package-list@https\:--archive.raspberrypi.org-debian.service/Packages
To fetch package lists every day at a random, but consistent, time between midnight and 6a.m. (local time):
systemctl enable --now fetch-repo-package-list@$(systemd-escape https://archive.raspberrypi.org/debian).timer