Pip itself is not responsible for finding the compiler. It will delegate the task of building the package to a build backend that is defined by the package being installed, in the [build-system] table of its pyproject.toml file. You should read the pyproject.toml in the source distribution of the project you’re trying to install (you can download the sdist, which is a .tar.gz archive, from the “Files” tab of the project page on PyPI).
(If the project does not contain a pyproject.toml file, then Pip will fallback to the setuptools build backend for backwards compatibility reasons.)