You actually can use a Bluetooth keyboard with a smartphone, but I have never done it and have no plans to, because carrying around a bulky keyboard defeats the purpose of a pocketable phone.
There are ways to use either a Mac or a Windows PC to read, send or respond to text messages. It's especially easy for those who use both a Mac and an iPhone. Apple has a help page called Set up iPhone to get SMS messages on Mac, that lists the four simple steps to forward text messages from the iPhone to the Mac.
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