"Thank you" pages and emails are set in registration workflows and are used as registration feedback.
The Thank You page, and the Thank You email are displayed and shown once a participant completes the registration form.
To add a thank you page or email to your event, you first need to create them in their respective Website, or Email Marketing module.
For more details on how to do that refer to this article: The Email/Website builder
Examples of both have been added to the bottom of the article.
After you have created the Thank You page or email enter the Workflows module of your event and find the Registration Workflow by the Template Name column:
Inside of the workflow you can assign your website and email to their corresponding sections:
Example of a thank you Email:
Example of a thank you Page: