If you don't mind, a shameless plug:

https://drakerossman.com/blog/nixos-for-apt-yum-users-a-gift...

I am doing a similar series of NixOS-related stuff, currently got only a single article out, yet quite "meaty".

You may want to check it out for a more streamlined approach in learning nix/NixOS which also compares it to familiar concepts from other Linux distributions. Will be releasing way more complete articles in the nearest future, hopefully within the next 2-3 weeks.