I want to use graphics.h according to this tutorial: Basic Graphic Programming in C++, which states that I have to install build-essential, an apt package. The equivalent pacman package seems to be base-devel, which does not seem to install graphics.h.
Where should I be able to install it?
If you want to draw things on the screen in C++, use SFML (sfml package) (or SDL 2 the very least), both are properly packaged for Arch Linux / Manjaro. If you want to create graphical user interfaces, check out Qt, wxWidgets, GTK.
“BGI” (which graphics.h is a part of) is the Borland Graphics Interface, it is old, and it is not and has never been available for Linux.