Most project just use the Git history along with LICENSE.txt file in the top level directory. The Git history is the most accurate record of who did what on a project. Some project keep a list of authors at the top of the file, but that get outdated pretty quickly if you aren't really pedantic about it. It also fails to record exactly what part of the file they worked on.
If you use external libraries, leave them in their own directory with their own license files, or at the very least leave them in their own files. So it's clear that they aren't part of the project and covered under their own terms.
If you start copy&pasting code snippets, it gets complicated, as now you might be mixing licenses, which might not even be compatible with each other.
They recommend a software-specific license instead, as those licenses handle code distribution requirements much more clearly.