There are ways to send iMessages programmatically. Apple does check for spam, but it’s not foolproof. And of course, it won’t help you against a targeted attack.

How exactly does any of this prevent people from sending spam to you?

I can count the total automated iMessage spams I've received on one hand. I can't do that with automated SMS spam I received in the last 24 hours.

So yes, not foolproof.

If there are ways, please share any documentation you have on it. I haven't found anything useful, myself.

I didn’t research it that much, but a good place to start would be AirMessage [1] or Mautrix [2]. Both of these require a Mac to work – it might work on a Hackintosh, though, or maybe using same tricks those forks of OpenHaystack use to run without a Mac (no pointers here, sorry). Hope this helps!

[1]: https://github.com/airmessage/airmessage-server

[2]: https://github.com/mautrix/imessage

Thank you.