Build pipelines I'm familiar with typically parse the includes from a .cpp file (and potentially recursively through the included headers) and resolve the includes to actual files based on include paths specified as part of the build configuration. Sometimes this information is cached in a .dep file or by some other mechanism. This information is then used to determine what actually needs to get built by an incremental build when a header changes.

The compiler is usually passed include paths to resolve includes when invoked. The build tool need to track this just to enable minimal incremental builds.