How does a telecom carrier such as AT&T or Verizon send an SMS message to a Twilio number?

I'm trying to further my understanding of how the SMS messaging system or 'texting' works.

Can someone explain how the back-end mechanisms work to deliver an SMS message from someone with an AT&T number to a number owned by Twilio?

This link Can a Twilio SMS number be sent via email to one of the big carriers? explains how a third-party provider can send "SMS messages" to your phone (via email), but I would like to know more about how the SMS messaging system works.

Thanks in advance.

Read over this article, provides some insights.

How One Company You’ve Never Heard Of Swallowed Tens Of Thousands Of Text Messages — Then Spit Them Back Out

AT&T, T-Mobile, and Sprint currently use Syniverse to route text messages to people on other networks, according to data available to Tyntec, a smaller messaging services company that spoke with The Verge. T-Mobile confirmed that it uses Syniverse, AT&T declined to comment, and Sprint did not respond to a request for comment. Verizon confirmed that it uses a competitor, SAP.