Does anybody know where the instructions and where the data is stored in the TI84+?

And is the control of the keyboard and the display only through ports - can maybe someone explain me how it is working exactly ?

Thanks for your help!

That's an extremely broad set of topics. The top two resources I like as documentation for a beginner on those topics (assuming you mean the monochrome TI-84 Plus, rather than the color-screen TI-84 Plus CE):

• Learn ASM in 28 Days: introduction to z80 assembly programming and the TI-83 Plus/TI-84 Plus, including an explanation of how port access to the keyboard and the LCD work.
  
• WikiTI has extensive documentation on memory location equates, BCALL (system call) locations and functionality, hardware details, etc.

What are you trying to figure out specifically?

I have to give a short description of these questions for university and I am not quite sure if I understood that completely.

It sounds like you need to read the sources and answer the questions yourself, then! Did you professor give you other resources to review? What class is this?

No we didn’t get any resources (it’s an IT- class)
Is it correct that the display and the keyboard can be controlled via Ports (IN and OUT- Instructions) and memory cells (LD instructions) ?

Or is only one of these options correct for the display/keyboard ?

Yes, on the monochrome TI-84 Plus, it's accurate that the display and the keyboard can be controlled by IN and OUT instructions: how to do it for the keyboard; how to do it for the LCD. I'll leave it as an exercise for the reader whether you can control the display and keyboard on the TI-84 Plus via memory cells (LD instructions).

Is there a page about this class online somewhere? I'm intrigued who might be teaching a class that includes a section on graphing calculators as simple computers.

Thank you!