Here are two examples:

Vanilla Flakes

{
  description = "Minimal multi-branch example";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-24.05";
    nixpkgs-unstable.url = "github:nixos/nixpkgs?ref=nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs =
    inputs@{ self
    , nixpkgs
    , nixpkgs-unstable
    , flake-utils
    , ...
    }:
    let
      pkgs = import nixpkgs {
        system = "aarch64-darwin";
      };
    in
    {
      upkgs = import nixpkgs-unstable {
        system = "aarch64-darwin";
      };
      packages."aarch64-darwin" = pkgs;
    };
}

Flake Utils

{
  description = "Minimal multi-branch example";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-24.05";
    nixpkgs-unstable.url = "github:nixos/nixpkgs?ref=nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs =
    inputs@{ self
    , nixpkgs
    , nixpkgs-unstable
    , flake-utils
    , ...
    }:
    let
      supportedSystems = [
        "x86_64-linux"
        "aarch64-linux"
        "aarch64-darwin"
      ];
    in
    flake-utils.lib.eachSystem supportedSystems
      (system:
      let
        pkgs = import nixpkgs {
          inherit system;
        };
      in
      {
        packages = pkgs;
      }) //
    flake-utils.lib.eachSystemPassThrough supportedSystems
      (system: {
        upkgs = import nixpkgs-unstable {
          inherit system;
        };
      });
}

In the Flake Util example, I'm able to build a multi-platform flake that exposes upkgs for each architecture.

Can anyone help me with recreating this minimal Flake Util setup in flake-parts ?