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A stock Ubuntu Server install downloads and applies updates on a timer. On a host machine, an update that runs during a rental can restart a service or replace a driver under a running instance, which interrupts the client and counts against the machine. Updates on a host machine must be applied in a maintenance window instead.

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Confirm automatic updates are off

Two APT settings and a timer control this. Check all three together. Query APT for the resolved value rather than reading a config file, because more than one file in /etc/apt/apt.conf.d/ sets these and the highest-numbered filename takes precedence:
A stock Ubuntu Server install returns the following, and every value is the opposite of what a host machine requires:
Both settings should read "0" and both units should report disabled.
cat /etc/apt/apt.conf.d/20auto-upgrades is not a sufficient check on its own. update-notifier-common ships 10periodic, which also sets APT::Periodic::Update-Package-Lists, so deleting 20auto-upgrades leaves the machine refreshing package lists on a timer. apt-config dump reports the merged result of every file.A setting that no file sets prints no line. This is equivalent to "0", because APT defaults both settings to off.
systemctl is-enabled unattended-upgrades on its own is also not a sufficient check. That unit is the shutdown-time helper. The daily run is driven by apt-daily-upgrade.timer and gated on the APT settings above.

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Turn them off

20auto-upgrades sorts after 10periodic, so writing both settings there takes precedence over any lower-numbered file:
tee echoes the file back, and systemctl reports the symlinks it removed. Run the check from the section above again to confirm the result.
This stops the machine installing updates on its own. It does not affect apt itself, or any upgrade you run manually.

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Updates still have to be applied

The machine no longer patches itself. Keeping the kernel at the latest security patch level for your Ubuntu release is a verification requirement, and a kernel update does not take effect until a reboot. When an update is available, schedule a window with vastai schedule maintenance so clients are notified, then follow Upgrade the Kernel, which covers checking what is available, both upgrade paths, and the NVIDIA driver steps required afterwards.