Built-in functions in V8 come in different flavors wrt implementation, depending on their functionality, performance requirements, and sometimes plain historical development.
Other functions are implemented as built-ins, which themselves can be implemented in a number of different ways. Some are implemented directly in platform-dependent assembly. Some are implemented in CodeStubAssembler, a platform-independent abstraction. Yet others are directly implemented in C++. Built-ins are sometimes also used to implement pieces of glue code, not necessarily entire functions. For a list, see src/builtins/builtins.h.