How to set up 404 correctly so that the site does not come up with urls that do not exist?

Question

htaccess

Good day to all.
I can't find any error.
I have a site in php.
.htacces and index.php manage redirects and other rules
like this index.php looks like

<?php

mb_internal_encoding("UTF-8");

$action = ''; 
$arguments = explode('/', $_SERVER['REQUEST_URI']);
$action = "default";

foreach( $arguments as $key => $value ){ 
     if (strpos($value, "utm_")) { 
         unset($arguments[$key]); 
     } 
 } 

if (count($arguments) > 1) {
  if (!ctype_space($arguments[1]) && !empty($arguments[1])) {
    $action = $arguments[1];
  }
}


if (count($arguments) > 2) {
  $id = $arguments[2];
}


if ($action == "default") {
  $pageTitle = "Дизайн интерьера в Киеве под Ключ. Скидка 10% ";
  $pageDesc = "Мы создаем дизайны интерьера, которые радуют наших заказчиков.  Наши дизайнеры интерьера справятся с любой поставленной задачей в кратчайшие сроки и под Ваш бюджет!";
  $pageKeys = "дизайн интерьера киев выгодно ремонт квартир";
  $idform = "Заявка с главной страницы";
  $headerform = "Обсудить дизайн проект с дизайнером";
  include("index_page.php");
}

else if (strtolower($action) != "default"){
  $pageTitle = "Страница не найдена |";
  include("404.php");
}

and this is what htaccess looks like

DefaultLanguage ru
AddDefaultCharset utf-8
php_value default_charset "utf-8"
php_value date.timezone "Europe/Kiev"

#AddType application/x-httpd-php .php .phar
RemoveHandler .html .htm
AddType application/x-httpd-php .php .htm .html .phtml .phar


<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteCond %{REQUEST_URI} !\?
RewriteCond %{REQUEST_URI} !\&
RewriteCond %{REQUEST_URI} !\=
RewriteCond %{REQUEST_URI} !\.
RewriteCond %{REQUEST_URI} !\/$
RewriteRule ^(.*[^\/])$ /$1/ [R=301,L] 

RewriteCond %{THE_REQUEST} //
   # Проверяем, повторяется ли слеш (//) более двух раз.
RewriteCond %{QUERY_STRING} !http(s|)://
  # Убедимся что это не урл в  GET
RewriteRule .* /$0 [R=301,L]
   # Исключаем все лишние слеши.
   

#################разрешаем фейсбуку разметку ссылок для рекламы блять##############################################################
 RewriteCond %{QUERY_STRING} ^(.*)(?:^|&)fbclid=(?:[^&]*)((?:&|$).*)$ [NC]
 RewriteCond %1%2 (^|&)([^&].*|$)
 RewriteRule ^(.*) $1?%2 [R=301,L]
#############################################################
  RewriteCond %{QUERY_STRING} ^(.*)(?:^|&)gclid=(?:[^&]*)((?:&|$).*)$ [NC]
  RewriteCond %1%2 (^|&)([^&].*|$)
 RewriteRule ^(.*) $1?%2 [R=301,L]

#RewriteRule ^/tmp/([^\.]+)\.(png|jpg|gif)$    /image.php?file=$1.$2 [NC,L]
RewriteCond %{REQUEST_URI} !^([\d\w\-\/\:\.]*(?:\.jpg|\.jpeg|\.png|\.ico|\.tif))$ [NC]
RewriteCond %{REQUEST_URI} !^([\d\w\-\/\:\.]*(?:\.css|\.js|\.eot|\.svg|\.ttf|\.woff|\.woff2|\.map))$ [NC]
RewriteCond %{REQUEST_URI} !^([\d\w\-\/\:\.]*(?:\.pdf|\.txt|\.html|\.xml))$ [NC]
#RewriteCond %{REQUEST_URI} !^([\d\w\-\/\:\.]*(?:\.phar|\.php))$ [NC]
RewriteCond %{REQUEST_URI} !^([\d\w\-\/\:\.]*(?:\.ogm|\.ogv|\.ogg|\.webm|\.mp4))$ [NC]

 RewriteRule . index.php  [L,QSA]  
 
 AddType video/ogg .ogm
AddType video/ogg .ogv
AddType video/ogg .ogg
AddType video/webm .webm
AddType video/mp4 .mp4

ErrorDocument 404 /404.php

</IfModule>
<IfModule mod_deflate.c>
  AddType image/svg+xml .svg
  AddOutputFilterByType DEFLATE image/svg+xml  
  AddOutputFilterByType DEFLATE application/rss+xml
  AddOutputFilterByType DEFLATE application/vnd.ms-fontobject
  AddOutputFilterByType DEFLATE application/x-font
  AddOutputFilterByType DEFLATE application/x-font-opentype
  AddOutputFilterByType DEFLATE application/x-font-otf
  AddOutputFilterByType DEFLATE application/x-font-truetype
  AddOutputFilterByType DEFLATE application/x-font-ttf
  AddOutputFilterByType DEFLATE application/x-javascript
  AddOutputFilterByType DEFLATE application/xhtml+xml
  AddOutputFilterByType DEFLATE application/xml
  AddOutputFilterByType DEFLATE font/opentype
  AddOutputFilterByType DEFLATE font/otf
  AddOutputFilterByType DEFLATE font/ttf
  AddOutputFilterByType DEFLATE image/svg+xml
  AddOutputFilterByType DEFLATE image/x-icon
  AddOutputFilterByType DEFLATE text/css
  AddOutputFilterByType DEFLATE text/html
  AddOutputFilterByType DEFLATE text/javascript
  AddOutputFilterByType DEFLATE text/plain
  AddOutputFilterByType DEFLATE text/xml
  AddOutputFilterByType DEFLATE image/svg+xml
</IfModule>
<IfModule mod_expires.c>
  ExpiresActive on
  ExpiresByType image/jpeg "access plus 1 year"
  ExpiresByType image/svg "access plus 1 year"
  ExpiresByType image/gif "access plus 1 year"
  ExpiresByType image/png "access plus 1 year"
  ExpiresByType text/javascript "access plus 1 year"
  ExpiresByType text/css "access plus 1 year"
  ExpiresByType application/javascript "access plus 1 year"
  ExpiresByType application/vnd.ms-fontobject "access plus 1 year"
  ExpiresByType application/x-font-ttf "access plus 1 year"
  ExpiresByType application/x-font-opentype "access plus 1 year"
  ExpiresByType application/x-font-woff "access plus 1 year"
  ExpiresByType image/svg+xml "access plus 1 year"
</IfModule>
  <IfModule mod_headers.c>
  <filesmatch "\.(ico|flv|jpg|jpeg|webp|png|gif|css|swf)$">
  	Header set Cache-Control "max-age=31536000, public"
  </filesmatch>
  <filesmatch "\.(html|htm)$">
  	Header set Cache-Control "max-age=7200, private, must-revalidate"
  </filesmatch>
  <filesmatch "\.(pdf)$">
  	Header set Cache-Control "max-age=86400, public"
  </filesmatch>
  <filesmatch "\.(js|otf|ttf|woff|woff2)$">
  	Header set Cache-Control "max-age=31536000, private"
  </filesmatch>
  </IfModule>

so no matter how much I suffer, I don’t understand why pages like:
https://goodidea.kiev.ua/& give 404
and pages like:
https://goodidea.kiev.ua/contact/%D1%84%D1% 86%D0%B...
just get a slash at the end.
I want pages to end with slashes.
But it turns out that
https://goodidea.kiev.ua/contact/
https://goodidea.kiev.ua/contactfsfefes
will simply live and a slash will be added to them.
https://goodidea.kiev.ua/contactfsfefes/
and so in my index.php file there are pages that exist. But the server does not process this rule.
I want only existing pages to have a slash added to them, and "invented" pages give me a 404.
Where is the error?