How do you write code that you cannot run?

I am a junior Software developer in a small company.

The thing I am asking is, how are you supposed to write code that you can not actually run yet?

There is a framework which I am supposed to extend. It is a bit documented and the codebase is not too big yet. It is not runnable yet. I just struggle to commit code for a new adapter. My main problem is not knowing if my code is runnable as it is. The project manager said to just commit the code. But how can you start fixing code when you already wrote five adapters afterwards?

I guess this is what software development is about. Adapting and just trying. I feel like I am building a car without the proper knowledge, where I just start the inspection of the different parts once the car is built and the gas tank is filled.

I don't need to test everything. But at least running the whole application once would be nice. The problem is, that there are many dependables which would need to be compiled as well. But for that the project has to many construction sited going on.

Any help on what mindset is needed here or the like?

Thanks in advance!