And how was std:copy implemented?

Your answer is valid only for a programming language which assumes that the standard library is implemented in another language.

C/C++ are supposed to be languages in which any program can be written, unlike languages like Java or Python.

Yes, C++ has those parts. You can use them to write something like std::copy. But use them, once, to write std::copy, and get that one implementation right (or have the library writers do it for you), and then use std::copy everywhere, rather than using void* everywhere. This makes your program much more type safe and less buggy, while still letting you write the parts where you really need to use the down-and-dirty stuff.

You cannot implement modern C++ in pure C++ nowadays, as the standard library requires compiler intrisics, just like a language like Java.