The backend of the LTB is based on the Symfony PHP Framework. See the composer.json file to see which version is currently in use.
Documentation regarding Symfony and its used components can be found at https://symfony.com/doc/current/index.html. For certain packages dedicated documentation exists: just check the Github page in case you need it.
We recommend Symfony: The Fast Track as a resource if you want to learn the basics of Symfony within reasonable time. It shows what Symfony is capable of and touches most of the concept used by the LTB.
Make sure to install the PHP dependencies first by running the following command in the project root folder:
composer install
The easiest way to run the LTB is to use the docker development image. It will contain all PHP dependencies by default.