Using clang from a non-default location and setting path to c++ standard library headers

I have cloned and built the llvm repository using instructions on this page : Clang - Getting Started
I am using MacOS.

I am trying to use clang with cmd:

<path_to_llvm>/build/bin/clang -std=c++20 myfile.cpp

I wish to use standard library headers like iostream and c++20 headers like concepts in myfile.

What is the way to have clang find these headers ? What paths need to be set to where ?
Are these c++ standard libraries, part of the llvm-repo I cloned ?

Your program may rely on some system headers not provided by llvm. You can use system clang plus option -v to print its extra include paths with appropriate options.

Thanks. I need to use the latest libc++ aas well. And I ran into errors installing the libc++ that comes along with llvm repo. Have posted another question for that.