Some confuses about Makefile

I have some confose about the process of execute on .h and .cpp .

It assign two functions without content in .h, and supplement them separately on hello.cpp* and factorial.cpp.

However, in main.cpp, it only has a header file without contents.

The example is similar to most projects by C program like (lda-c).

In lda-c 's directory, there several .h whose .cpp that name after *.h.