As an app (extension) developer you should only use the Nextcloud APIs and nothing of the underlying Symfony. This layer of abstraction allows Nextcloud core developers update dependencies while apps continue to work.
So, again, don’t use the Symfony things directly. Read the manual and use Nextcloud APIs.
Okay @ChristophWurst
So, I suppose that the APIs are developed in Symfony ?
Anyway, if I will develop an extension Nextcloud, I read the documentation recommended by @stefan-niedermann
I apply for a FullStack Developer for a french company and they would developed extensions for Nextcloud and others projects.
Whence my opening of the subject
See them as libraries, not frameworks. We take the components that we need to build the software. It’s standard practice. Symfony for the main components, Doctrine for database access, Guzzle for HTTP client and so on.