How to return a 404 response code?

Question

CMS

On a site on some engine unfamiliar to me, a 404 error is not processed. What can I do to ensure that non-existent pages return a 404 response code?

Answer 1

header("HTTP/1.0 404 Not Found");
or more generic for Apache
.htaccess
ErrorDocument 404 /error.html

Answer 2

Something is wrong somewhere, what should I do?
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First, figure out where the page not found handler works, for this you will have to deal with a framework that is unfamiliar to you. Then add to this question specifically what you do not succeed))