In order to access the admin area of CiviCRM, you need to login through your website. In this case, since we're using Joomla, you'll log in through the Joomla Administrator login. In other words, click the big green button!
Are you checking to see if CiviCRM is compatible with Joomla 4? We're excited about Joomla 4 too and are actively working through integration issues.