PHPUnit
PHPUnit by Sebastian Bergmann is an advanced unit testing framework for PHP. It is installed as Composer dependency and is not part of Moodle installation. To run PHPUnit tests, you have to manually install it on your development computer or test server.
Unit testing for the Moodle App
Unit tests are written in JavaScript using Jest. If you want to create a new one, Jest is already configured and you only need to create a file ending with .test.ts within the project. If you're going to do so, remember to follow the file location conventions.