sitemap – In the case of 404 Page, the URL should be 404 or we can change the content to 404 and the URL stays the same

I'm confused about the 404 page

Suppose I have a landing page

https://www.example.com/testingpage

The page does not exist anymore. What should I do with it?

Case 1: Should I redirect the URL?

https://www.example.com/404

Or
Case 2: Should I simply replace the content with a 404 page?
URL stays the same
https://www.example.com/testingpage
Content:
404

Which case should be used