The difference is if you're running tests from your root project directory, the package is importable regardless of whether or not it is installed, as python picks up packages in the current working directory by name. src/ prevents this.
2 karma · joined July 28, 2022
Another good tool (which was endorsed by the PyPA) is Hatch - https://hatch.pypa.io/latest/environment/
I currently use PDM because it supports conda virtual environments for isolation, but am keeping an eye on Hatch.