I don’t know the details but both use services from the carrier. I think google hosts for the carriers but there are checks thats rely on the carrier. If you look at the details for google messages it address specific URLs based on the carrier.
For iPhone its all carrier depend. If the carrier doesn’t support it, iPhone will not have it.
I like Boox but they are on the more pricey side. Basically an android tablet with an e-ink display.