For example, in the code above you have two clear responsibilities, perform the sum and store the operation in a journal.
I would create a couple new classes.
A Calculator, responsible for performing the operations needed (Sum, Substract, Multiply...).
A Journal, responsible for keeping the journal.
The controller would then be responsible only to read the request, call this classes if needed, and build and return the response.
Following what I said in #6 try to separate the code present in CalculatorController in smaller classes that are responsible only for one thing.
https://github.com/pikachumm/CalculatorService/blob/21d5ad8f28f037db010e9aae5dcb3347b3757dba/CalculadoraServer/Controllers/CalculatorController.cs#L38-L53
For example, in the code above you have two clear responsibilities, perform the sum and store the operation in a journal. I would create a couple new classes.
The controller would then be responsible only to read the request, call this classes if needed, and build and return the response.
You can also use this to learn about Inversion of control / Dependency Injection and how we use it at Evicertia: https://www.martinfowler.com/bliki/InversionOfControl.html https://aspnetboilerplate.com/Pages/Documents/Dependency-Injection