To stop Fedora to automatically download and install system and software updates on restart?
dnf is a background service that updates the repository metadata automatically. dnf makecache is scheduled to run after every reboot
To disable dnf-makecache.service:
systemctl disable dnf-makecache.service
Also disable the dnf-makecache.timer or the service will be restarted:
systemctl disable dnf-makecache.timer