Sword is WordPress run by Symfony. Your WordPress code has access to all Symfony features, while your Symfony code can be WordPress-independent. Build your theme or child-theme just like you would build a Symfony app.
Login into WordPress will also log you into Symfony. Check the user WordPress capabilities using Symfony's authorization checker.
Sword comes with a preconfigured Docker environment that can be used both in development and production. WordPress is completely configurable through environment variables.