Methods to redirect to class web page if web page doesn’t exists?

In my present case, the system attempt to discover a web page and present 404 if it doesn’t discover.
I would really like with the identical url… like https://instance.com/{{slug}}/

1. Test if a web page with this slug exists :
   1. • Sure – present the web page
   2. • No – change to the class web page (with similar slug)
   3. • No class web page -> present web page 404

What’s the greatest recommandanted method to do that in WP?