First time i used let it was to inline variable declaration with assignment . Can’t remember the language.

Do the laws say you need to store this after verification? I mean a one time verification on account creation would work?

Here’s the wiki article - never had an issue following that

https://wiki.debian.org/UnattendedUpgrades

Can also suggest reading the man page. For completeness and just general knowledge also read the man pages for apt, apt.conf