SMS class 0

In the USA all carriers simply won’t process class 0 SMS. They are technically legal (at least in the USA) but can be abused (Google phone’s at one time were vulnerable to a flash SMS exploit, since patched) and annoying and serve no really useful purpose so carriers in the USA adopted the policy of simply blocking them on their networks or they strip the class 0 from them and send them as regular SMS texts.

There are apps in the App Store to send so-called joke class 0 SMS (and the developer charges to do it) but Apple’s rules for those apps to even be in the App Store are that they cannot be sent anonymously and they must include a user dismiss button and not disappear automatically (so the user knows they were pinged). And as I mentioned, in the USA those will actually end up being transmitted as just a regular SMS text anyway.

And you hardly need to research how it works. The class 0 messaging protocol is part of the global GSM standards. Just read the standards and related technical documentation to know how it works. It’s not like it’s some deep secret thing. SMS including the class 0 specs have been around for decades.

No

Because with the exception of things like emergency notifications, Flash SMS messages are illegal in most countries.

Not possible with Apple’s default software. You would need to install a specialist app and probably special permissions on your cellular service. Most networks don’t enable class 0 for regular users.