RCS is just a standard, much like how SMS is just a standard. Google’s Messaging app is just one implementation of it, though it probably is the most popular in the US it is not the only one, nor is Google able to decide who cannot use it. Carriers typically have their own, though you may have recently heard that T-Mobile decided to switch over to Google’s.

It’s nesting season

Probably an unpopular opinion on here, but the OS I recommend for grandparents and parents is ChromeOS. It’s so locked down that it’s almost indestructible, and they almost never need any specialized software that you’d use Windows/MacOS for. If you’re savvy enough you can also use Linux on it in a container, which is how I prefer to use it for day to day stuff (in my case, data related workflows).