makefiles and associations between .h and .cpp

Dear All,
We have been provided libraries to work with. If any change is required to any definition, I see that in the past, programmers have copied the .cpp file from the library folder into the project folder and added this as one of the source files in makefile; this makefile already contains the path to libraries. I wonder how the compiler picks the .cpp file from the project folder and not the library folder. Is the priority set by the order of appearance in the makefile?

Also, where can I get details about makefiles? We use two files for the build process: makefile.trs and myproject.mk. I could find details on how these 2 work in conjunction.

Thanks for looking,
Shivraman