Apt has a mechanism via a systemd timer (apt-daily.timer) to automatically download updates. By default the feature is turned off. To turn it on, you need to modify the APT::Periodic::Update-Package-List configuration variable.
Put the following line in /etc/apt/apt.conf.d/10periodic:
APT::Periodic::Update-Package-Lists "1";
Replace 1 with how often (in days) you want to fetch updates.
See Also
UnattendedUpgrades - Install upgrades automatically.