How does this work with an existing website?
As Jon says, CiviCRM is installed on top of WordPress, Drupal/Backdrop or Joomla, so if your main website uses one of those you have the option to add Civi to it.

Alternatively, you can leave the main website as is and install CiviCRM and one of the CMS's on a separate site.  Then for example, your main website has the write up of your event and includes a link for booking that goes to the Civi site to do the event booking and payment.