Errors in the program, a problem in the structure?

Question

C++ / C#

SOURCE

As far as I can guess, most of the errors are related to the very organization and location of the files.

Answer 1

Rather, with the fact that many #includes are missing. It's not like there's just one thing here.
For example, UserDB.h uses the User class, which is described in Auth.h. #include "Auth.h" will fix the first error.
Where string is used, it should also be #include . Moreover, you can’t just omit the name of the namespace
std::
In short, the code is not completed.