Why is there a index.php

Hi all,
I am new to CI for like half a year and working on my second project using it. I wonder about one thing:
My "normal" routing works well, domain.tld/page1 etc, Apache is set up to drop the index.php

See https://codeigniter.com/user_guide/insta...-site-uris

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper