.htacess to perform rewrite if file does not exist - .htaccess

Question

I have two separate .htaccess files that I'd like to be fused together so that the first rewrite always takes precedence, which redirects some traffic to https base which domain. Then if the file at the url does not exist, then it sends the traffic to a php file. It is a url shortener, but if that returns at 404, then it shows a 404 error page.
Here are the bits and pieces of the .htaccess files:
This, below, I believe should redirect all http traffic except kore.tt and korett.com to https:
RewriteEngine on
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} !^(www\.)?(kore\.tt|korett\.com) [NC]
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [NE,L,R=301]
This is something from the url shortener that is supposed to send traffic that doesn't exist to loader.php
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ /loader.php [L]
</IfModule>
But then if that returns a 404 error. Then this is the simple 404 error catch.
ErrorDocument 404 /404.html

Answer 1

You can use a condition in your htaccess. There are many more ways to create rules. See if this can work for you.
#check to see if loader.php exists on the filesystem, then do rewrite
<If "-f %{DOCUMENT_ROOT} . '/loader.php'">
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ /loader.php [L]
</If>
#otherwise redirect to 404 page
<Else>
RewriteEngine On
RewriteRule ^ /404.html [R=404,L]
</Else>