Symfony uses Propel as the ORM and PHP Data Objects as the database abstraction layer. It means that you must first describe the relational schema of your database in YAML before generating the object model classes. Then, at runtime, use the methods of the object and peer classes to retrieve information about a record or a set of records. You can override them and extend the model easily by adding methods to the custom classes. The connection settings are defined in a databases.yml file, which can support more than one connection. And the command line contains special tasks to avoid duplicate structure definition.
The model layer is the most complex of the symfony framework. One reason for this complexity is that data manipulation is an intricate matter. The related security issues are crucial for a website and should not be ignored. Another reason is that symfony is more suited for middle- to large-scale applications in an enterprise context. In such applications, the automations provided by the symfony model really represent a gain of time, worth the investment in learning its internals.
So don't hesitate to spend some time testing the model objects and methods to fully understand them. The solidity and scalability of your applications will be a great reward.