

Well, NixOS is mostly for enthusiasts and it’s very much the opposite of beginner friendly.
The idea is that you configure your system in a configuration file, then run a command that makes your system match exactly what you configured.
So instead of apt install
or similar you just add the package to your config, run a single command to rebuild the system and you’re done.
Which also means you’re mostly on your own, most guides for other distros don’t work and the documentation on how to do the things in NixOS are very incomplete. It’s nice and fun, but definitely not for an average user.
It’s not any more secure. The point that “installing random debs is insecure” has been running around for at least the last 16 years I’ve been a Linux user.
While it’s technically true, AppImages are as secure as random debs. Same with random repositories that are not provided by your system. Same with flatpaks.
And unless you’re an extremely basic user, you’ll eventually have to install an application not in your repositories. The method doesn’t really matter, it’s all equally (in)secure.