How can this be refactored?

Question

C++ / C#

https://github.com/stepan-neretin7/calc/
Hello! I write my calculator in C. There was a need to make an interactive mode. I realized that with my code it would add a bit of a pain, because everything is somehow connected.
Maybe someone can go through the code, give advice on how it can be all otrefachit.
Can you also explain, please, what strategy is followed in C for error handling