How to install packages come as flake, and other forms

Very nooby questions incoming

Somedays ago I wanted to install ACross (a v2ray interface) on nixos, as of usual I went to website to search is it available on nixpkgs or not, and it yeilded no result, but then I went to their website and they had a nixbuild, I asked for that package to be added in nixos and mentioned it seems like its built for nixos with a flake (sth that I have no clue of while I probably should)

also I requested for eric6 to be packaged and one user in github gave me this:

what is this? I get its somehow a build of eric in nix but how should I integrate it in my configuration.nix?
I also saw a nix flake which made yo code command work, and I have no clue how to activate it and what it means

Im wondering,

  1. How should I install ACross and Eric6 based on the links
  2. How to add flakes and what they are? instruction to build one version of a package or like a ppa in ubuntu?
  3. Why should they be used this commonly? (I read a bit about its philo on nixpills but I still don’t understand why we should package softwares in them instead of publishing them in nixpkgs monorepo)
  4. How to use Appimage and npm pip and stuff
  5. How to activate a flake, and what is nix develop? is there a developer mode?
  6. why should I run my keyboard backlight manager and many other executables with stearm-run, and what is the better way to do so? can I dockerize them to fix this issue? or can a special flake help me in this case?
  7. is there a way to install an AppImage on nix?

It probably should’ve been more than one post, but they are related in one fact, how to get the software working, and the tricks you use
my only core value in these is I want my system to be declarative if I could I would