I've been a Linux user for 25 years. You can reconfigure the OS to use additional repositories. It may or may not work, and only if there is a repository specific to both your distro and its version. But it's not a good idea.
In particular, OS upgrades are very likely to break. Being able to upgrade itself is a basic requirement of any modern OS. If your Linux distro corrupts itself on upgrade or fails to do so and you file a bug report you'll be told to remove any third party software because that's not supported.
This would be like if your Mac started crashing on boot because you downloaded a word processor from a website, and then Apple say "sorry, we only support apps coming from the app store". They don't do that, but Red Hat or Canonical will.
The stuff about Linux not letting you install stuff flies far in the face of like everyone’s knowledge of Linux. Your description of how Linux installation works is pure fantasy.