So SMS goes over the phone network, online chat programs (like Discord) go over their own servers obviously, but what does RCS and iMessage go through?
Google's and Apple's servers on the internet? The phone system, seeing they're SMS-based?
Also, if I'm in the SMS messaging app, I expect my messages to be SMS, not internet-based?
Messaging apps haven't been pure SMS for a long time. If you've sent photos/images, it's using MMS, which basically uploads the picture to a HTTP server hosted by your carrier and sends the URL in a SMS message that the receiving app knows to automatically download and display the image, provided you have a data connection.
iMessage is proprietary to Apple, so of course it goes through Apple's servers. When you set up your iPhone, it registers your phone number with Apple's iMessage server so that whenever someone tries to send a message to your number, it sends it to the iMessage cloud instead of SMS. This can actually cause problems when you switch from an iPhone to Android because other Apple users trying to send you text messages might still get routed to iMessage instead of SMS, and there's no iMessage available for Android. You may have to use a website that Apple provides to manually deregister the phone number from iMessage in that case.
I'm not all that familiar with how RCS works but presumably there's a similar registration process so that other phones know that your phone is RCS capable, except decentralized since it's not controlled by a single company.
Wow, Apple really manages to come up with every possible way to lock-in and hamstring competitors.
I didn't know how to resolve it at the time and it only got fixed for me after I replaced my phone and got a new phone number.
Before iMessage was so ubiquitous on iPhones, everyone just expected that you could message anyone else, regardless of their phone or carrier, over SMS/MMS. And I also don't fault Apple at all for releasing iMessage. But it's not like they released iMessage as just another app in the App Store like Whatsapp or Telegram. In true "Embrace/Extend/Extinguish" fashion, iMessage was just released as the default "texting app" on iPhones, and over the years the situation became more and more awful for iPhone -> Android communication. Non-US folks often don't understand the situation - "Why don't you just choose another messaging app?" Because in the US most people never "chose" a messaging app to begin with - they just used the default texting app on their phone. The US didn't really have the issue with exhorbitant SMS fees that other countries had that pushed usage to other messaging apps.
Here's the deregister tool, btw https://selfsolve.apple.com/deregister-imessage/
You can just tap to resend a message as sms which will send all future messages as sms.
P2P SMS is also quickly becoming obsolete in favor of OTT messengers like WhatsApp.
As for who runs the servers: iMessage goes through Apple's servers.
RCS ostensibly goes through a federated network of telco-operated servers. But since almost none can actually be bothered to implement it (they've given up on trying to monetize it, so they have no incentive at all for incurring additional cost deploying and maintaining them), Google runs it for them, via a company they acquired [1].
> Also, if I'm in the SMS messaging app, I expect my messages to be SMS, not internet-based?
Is your SMS app actually called "SMS", these days? I usually see it called "messaging" or similar. In any case, assuming these apps do RCS, it'll be Internet-based.