Copying from what I wrote in Build System Interface - pip documentation v23.3.2, “When dealing with installable source distributions of a package, pip does not directly handle the build process for the package. This responsibility is delegated to “build backends” – also known as “build systems”. This means that pip needs an interface, to interact with these build backends.” The build backend (eg: setuptools, flit-core, hatchling, poetry-core etc) do the actual work of generating the source distribution / wheel and providing it to pip, using the mechanism described in PEP 517