These are the docs for Directus 8, a legacy version of the platform. If you're looking for the current Directus 9 documentation, go here: https://docs.directus.io
Directus runs on PHP and MySQL, meaning it can be installed in numerous different ways and environments.
Pulling the codebase from git ensures that you can easily upgrade to newer versions in the future.
Has everything you need to get started; no need to install, configure, or manage additional packages on your server.