In both popular HTTP servers - apache and nginx you could easily set a HTML page or even a script for any HTTP error, including 404 File not found one. Obviously you could write anything in it, from just "File not found" to "Reptilian thought police removed this file for thought crime".

Meanwhile custom handler for 404 error used in some CMS to generate pages on the fly using path as parameter for generation. There is no real file named, say https://example.com/some/file/name.html on server, 404 error handler generate contents instead of displaying 404 error.

Your screenshot of 404 error mean absolutely nothing. Any random path on qalert.net will show it, like https://qalerts.net/random/stuff/as/page/name.html