The LeafOS website runs on Symfony 6.2. You can check the Symfony documentation for the framework requirements. There are no additional requirements other than those.
After the requirements have been met, you can install the required dependencies by running composer install.
There are two ways to run the site locally: by using PHP built-in webserver or by using Apache/Nginx.
After you've installed the required dependencies, you can start the PHP server by running the following command from the project root:
php -S localhost:8000 -t public/
After that, you can check the site by going to localhost:8000 on your browser. If the port 8000 is in use in your machine, simple change it to something else.
Both Apache and Nginx are supported by Symfony. There is a dedicated section on Symfony documentation for that, please refer to this page in order to configure Apache/Nginx.