How does basic compilation work for an arduino project ?

i use original "serial" port code , change things in there, putting in extended error reporting.
i located "hardware serial" .h , .cpp files in arduino folder.
am wrong in assuming changes make in these reflected in new project file compliled ?
i thought if assumption incorrect, there may other way core code project obtained can't life of me think how.
so, in summary, .h , .cpp files in original core libraries examined compliler , re-compiled every time project compiled ?

it compiles them once when compile first time after launching ide. after that, recompiles if files have changed since last compiled them.

it's better idea copy hardwareserial.{h,cpp} under different name libraries folder don't have keep several versions of core.

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