I'm not an Android user, so pardon me if this is a stupid thing to say, but it's weird to me that these phones apparently have some new UI unique to them. I thought Android was just Android. Won't other Android phones get this update?

Android is not just Android. The device vendors have to customize it to fit their devices by including drivers for example. Device vendors have the option to change the look pretty heavily, Samsung TouchWiz was infamous, Chinese vendors also offer very customized versions, including making it look like iOS. What you are seeing is material design 3 "expressive" which will be rolled out in the next minor Android version and Google apps

Not all phones capable of running Android (everything?) have the hardware to host the local LLM models at a useful level of performance.