Every form ends with a thank-you page. Insert a Page break at the end of the form and switch on 'Thank you' page in its settings, then add any text, headings, images or embeds below it. Mention answers with @ to personalise it ('Thanks @First name!'); calculated variables can be mentioned the same way, which is how quizzes show a score.
The default thank-you title and message can also be changed from Settings, and Redirect on completion sends respondents to a URL instead of showing the page.