Psr4 autoloading

Drupal 8 implements the PSR-4 standard for package-based PHP namespace autoloading by the PHP Framework Interoperability Group. Autoloading works for both modules and themes. In order to write custom modules in D9 , there has to be followed a specific directory structure and notation. Example sample.module directory structure. Some basic rules are -

• Each module has a namespace that corresponds to its module name.

Drupal\sample

• The module's namespace is mapped to the ./src/ folder in the module directory.

Drupal\sample\ → modules/sample/src/

• Anything after the module namespace directly maps to the directory and file structure in the ./src/ folder.

Drupal\sample\Entity\Tomato → modules/sample/src/Entity/Tomato.php

• The identical logic applies to PHPUnit tests contained in ./tests/src
• The modules/sample/src/Tests folder contains the SimpleTest test code.
• The modules/sample/tests folder contains the PHPUnit test code.

More details can be found at D.o psr4 documentation page