How do I troubleshoot undelivered SMS messages to non-Apple phones on newer iPhones and iPads?

Sending a message to a non-Apple phone I assume. The message come back undelivered. Now this is the important part “ Apple help is so old that the details and the steps do not exist for newer phones or iPads. Possibly a purge is required. Apple make the customers get new devices but the help data is for old devices. WTH

ROBIN