The Credential Source can be stored the way you want. As the Webauthn\PublicKeyCredentialSource class can be converted into JSON, it could be stored in a filesystem.
In general, Symfony applications use Doctrine. That is why the bundle provides a way to use Doctrine as storage system.
The Doctrine Entity
Hereafter an example of an entity.
This is the most simple example. Feel free to add custom fields that fits on your needs e.g. created_ator is_revoked.
To ease the integration into your application, the bundle provides a concrete class that you can extend.
In this following example, we extend that class and add a method to get all credentials for a specific user handle. Feel free to add your own methods.
We must override the method saveCredentialSource because we may receive Webauthn\PublicKeyCredentialSource objects instead of App\Entity\PublicKeyCredentialSource.