Installation procedure is entirely automated; there is an installation documentation you should rely on.
When you are using the source code, there are extra steps, to get all third party libraries installed.
First, download and install composer, a PHP dependency management tool. Second, download and install npm, a JS dependency management tool. Once done, go to the GLPI directory and just run:
$ php bin/console dependencies installhttps://glpi-project.org/downloads/
git clone https://github.com/pluginsGLPI/ocsinventoryng
git clone https://github.com/fusioninventory/fusioninventory-agent