I've been trying to find a C/C++ framework for apps on macos. I couldn't find good docs on metal. Is there a way to write C++ apps without any other library?
If you insist, Qt is a cross-platform C++ framework for app development which can target macOS.
However, I think you're rowing against the tide here.
Current documentation and online articles about macOS development lean towards Swift (ten years old) and SwiftUI (five years old), and Objective C. You can use C++ with ObjC code, just change the file extension from .m to .mm.
I don't think it is possible to write any meaningful app without using a library of some sort. What exactly are you trying to avoid?