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.

Whatever you can afford for pixel 8 and above, if possible. Those will be supported for longer.

Isn’t matrix more like slack that you are looking for?

Yes to google maps if you have the play services activated.

No to pay/wallet.

Yes to Android Auto as of recently. The grapheneos team just released a implementation of it.