Initially Codeception was a wrapper for PHPUnit with additional test formats and modules. Test orchestration and reporting was performed by PHPUnit code.

This is no longer the case in Codeception 5, the role of PHPUnit was reduced to provider of assertions, also PHPUnit TestCase test format is supported. We use a few utility classes too - PHPUnit\Runner\Version class to implement compatibility with PHPUnit 9 and 10, PHPUnit\Util\Test to get linesToBeCovered and LinesToBeUsed and we catch and throw a few PHPUnit exceptions.

We use phpunit/php-code-coverage and sebastian/diff libraries too, but Codeception 5 is no longer powered by PHPUnit, we have our own engine :)