The easiest way to run Pixelfed for development on Mac is using Laravel's Valet. Valet contains a number of tools to run the application locally with minimal fuss.
You'll need to install Homebrew, a Mac package manager, to start off. Once you've installed Homebrew, if you haven't already installed PHP 7.3, install it with brew install php.
After installing PHP, you'll want to install Composer, which is a PHP dependency manager that can also install Valet for you. Then run composer global require laravel/valet, which will install Valet as a globally-available program. To access it, be sure to add ~/.composer/vendor/bin to your shell's PATH variable. Then run valet install to finish installing Valet's dependencies.
Running the valet park command inside a folder will allow you to access [subfolder].test as a local domain. You can set this up however you wish, though an