nixos

I've been messing around with nix flakes to configure my NixOS system and to put the final touches on I wanted to write an overlay of Neovim with the plugins and everything configured with Lua and then use this overlay to install neovim-qt. After reading various docs on Overlays, I get the fundamentals but I don't get where to look to find a list of all the configurable options that an overlay provides? Typically I can look in the home-mananger nix decleration of a package for the options exposed or nixpkgs for the options in configuration.nix. Any help that will dive deeper into overlays or where to find their options would be greatly appreciated!