how to show a specific post for 404 wordpress

Honestly this should be a lot easier than actually is. The simplest way is to implement a 404.html page on your theme/templates folder but that means a static and hardcoded approach. Furthermore you might be using a language plugin like Polylang and you’d like to show different posts translations depending on the present language.

Most commun but not correct approach

A common solution is to use the filter template_redirect , this is not a good idea because what we want is to change the WP_Query selected Post and if you do that on the template_redirect filter it might be overwritten later in the script.

The best way to show a custom wordpress post when a 404 happens is using the add_filter('404_template') and then create a new WP_Query instance pointing to the Post you’d like to show.
First create the WordPress Post you’d like to use and then use the 404_template filter

Example of code below:

add_filter('404_template',function($template, $type, $templates){
	    $request_uri = $_SERVER['REQUEST_URI'];
	    if (stripos($request_uri, '/wp-content/uploads/') === false) {//this check is done because if a file like na image is not found the 404_template filter is called
		    global $wp_query;
                    $not_found_page_id = 1;//this is an example, if you are using Polylang you can use pll_get_post(1);
		    $wp_query = new WP_Query([
			    'post_type'=>'page',
			    'p'=>$not_found_page_id
		    ]);
	    }
	    return $template;
},3,10);