Problem..

The 404 page is only displayed if the requested page does not exist.

Are you trying to ask “Why is something displayed no matter what I enter?”

I just checked your site, I understand your problem now.
I will see what I can find.

I am Canadian

Your theme needs to have a 404.php file.

https://codex.www.ads-software.com/Creating_an_Error_404_Page

I have the statick page on link https://arys-my.name/?p=18

Actually, that is NOT a static page – that’s a post.

(if WP doesn’t have a 404.php template files) it always displays the main/front page of the blog in case of an error/non-existent file.

[I am a Canadian, too – but originally, a Transylvanian]

My template have 404.php

Your site is NOT generating a 404, which is why the 404 page does not show. If you are using any sort of redirection plugin, disable it and contact your web host about this problem.