Typo3 with non-root domain

I’m a newbie to Typo3 (v 12.2.0) and got lost just before configuring the backend.

I have a domain example.com and Typo3 should be accessible via (https://)example.com/typo3test1. The “https://” is automatically inserted by Apache2 (v2.4.54). “typo3test1” is just a name to remember that it is a test installation.
Please note: my installation is for a non-root domain.

Installation was done successfully by entering example.com/typo3test1 in the browser, that is, a new MariaDB for Typo3 was created and I was asked if I want to proceed to the backend configuration.

The URL was changed automatically to

followed by a note:
The requested URL was not found on this server.

Entering only
the login menu appears, I fill in the login data, and then the same error message appears.

Could somebody point me to the (possible) cause of the problem, please?