So is there such a repository or not ? Is every nixos user making the configuration file from scratch in vim ?

That's how you should do it IMHO. You will miss out on a lot of benefits of NixOS if you don't understand how the config system works, at least to the point of writing your own config files.

I figured there should be a repository where most user just pick a ready made file and that’s the end of that. Not really having to learn the syntax of that file and writing it down ?

There are lots of examples online that can get you inspired, but ultimately you are building your own system and it's up to you to write the config. Feel free to copy code from other people, as long as you follow the licenses in the projects (a lot are CC0/public domain, so you can just steal stuff freely with no remorse, but it's also nice to mention the original author)