concat is an intrinsic, in other words, there is no real source for it, as the compiler generates code on the fly.
Depending on argument types, there can be several different RTL helpers involved, or even non at all.
Most common Intrinsics are declared for documentation purposes in rtl/inc/system.fpd. If a function is in there, it is not a "normal" function/procedure.
Afaik concat uses helpers with "concat" in the declaration in rtl/inc/sstrings.inc for shortstrings, rtl/inc/astrings.inc for ansistrings and rtl/inc/ustrings.inc for unicodestrings etc.