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.
It is up to you to create a credential source repository. This service shall implement Webauthn\Bundle\Repository\PublicKeyCredentialSourceRepositoryInterface.
Registration Capability
By default, the User Entity Repository is not able to register any user account. You can add this behaviour by implementing the interface Webauthn\Bundle\Repository\CanRegisterUserEntity.
Doctrine Repository
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_at or 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\WebauthnCredential.