I know that Pixel 7's and above support it. There are Reddit posts showing they have the feature already. Satellite messaging is just using standard 4G/LTE from Starlink. I wouldn't be surprised if this is only an OS update away for most newer phones.

Correct. It needs hardware and will only be on flagships, if the OEM includes it..

Another nice feature on the latest iPhone is the UWB chip egg even the older models have but now you can find another iPhone 15 user in a crowd if they share their location. The UWB chip will guide you right to them

I've wanted something like this for at least a decade for when my wife and I get separated in the Mall so that instead of calling her to see where she is (and she often doesn't hear it ringing) I'd be able to just use the phone to lead me to her... Pretty useful in real life. And it also works for your misplaced tracker tile, air pods, iPad etc

https://www.androidauthority.com/how-to-get-gemini-nano-on-pixel-8-8a-3450466/

Pixel 8 pro already does. I'm not sure, but I think Samsung has something too.

I doubt it. The Pixels may be doing some but with Google I'm sure it's not much as they always prefer server side computing.

iPhone will definitely aim to do most of it on device and use the server as little as possible. Which imo is the way to go.