TypoScript by concept is meant for frontend context only. There is TSconfig for backend as well. Extbase introduced a way to also configure backend modules via TypoScript. Nowadays TypoScript is also used in other context, e.g. scheduler tasks or commands.
I'll explain how the actual TypoScript is loaded, so you understand that part and can use it.
Since TYPO3 10.0 it is possible, actually necessary, to use FQCN (=Fully Qualified Class Names) for controllers when configuring plugins and modules. This provides a new benefit in terms of reusing and sharing code between different extensions.
This blog post will explain the benefit and provide an outlook in a possible future of TYPO3 Extbase extensions.
TYPO3 provides a way of dependency injection. This way you do not need to resolve dependencies inside of your code, but the framework will resolve and provide the dependencies for you. This is provided by the framework Extbase, back ported of Flow.
The main benefit is the flexibility. Using Interfaces to define dependencies, instead of concrete classes, it’s possible to exchange injected dependencies just by configuring the framework. This way you can exchange classes in 3rd party code and receive a huge flexibility. Same goes for testing your code. In this Post I will show you the different ways to make use of dependency injection inside of TYPO3 and provide help for edge cases.