In Debian based systems, like Ubuntu, the package responsible for automatic updates is called
If you want to stop automatic security updates to the system, a simple way is to uninstall it, as in:
sudo apt remove unattended-upgrades
From `man unattended-upgrade'
unattended-upgrade - automatic installation of security (and other)
This program can download and install security upgrades
automatically and unattended,
taking care to only install packages from the configured APT source, and checking for dpkg
prompts about configuration file changes. All output is logged to /var/log/unattended-
This script is the backend for the APT::Periodic::Unattended-Upgrade option and designed
to be run from cron (e.g. via /etc/cron.daily/apt).
As for rebooting, it is not mandatory rebooting after upgrades. You can keep using it. However if you want to keep running it over time, the system is generally more happy if you do not delete the packages of the version of the kernel that is running.
Obviously from the point of view of security, it is advised to update packages, and reboot periodically when there is a new kernel update to be running a new kernel version.
You also have some Linux versions were the kernel is patched on run-time, and you rarely need to reboot.