If you put a modified version of one of the core files (I assume from framework-arduinoteensy), putting that file in to lib/ won’t do what you want, because the original file still exists and would also be compiled in (-> double definition). Also you probably didn’t have the right folder structure if you put it directly in lib/ (but doesn’t matter).
See related topic How to modify Teensy core files? and especially Custom framework? or platform? - #2 by maxgerhardt (mentally replace framework-arduino-nrf52-mbedos with framework-arduinoteens, same procedure).
Further see documentation.
See documentation and documentation.