I'm not sure I fully understand your question. I assume that you want to provide a normal HTTP response if the client uses the correct URL and the default page (with status 200) when the client uses a non-existent URL.
If so, this can be achieved as follows:
RewriteCond %{REQUEST_FILENAME} !-f RewriteRule ^.*+ /dummy.html
The first line is the condition that the URL does not process an existing file on the web server. If this condition is met, the second line is executed, which serves to the dummy page to the client.
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