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'm not from the south and use "y'all" all the time. Find it very useful for filling in a gap that English has and slightly faster than saying "you all". Its gender neutral in my opinion.
Never once thought of it as offensive.