About android : What-kind-of-softwareapp-is-the-Google-Nest-Hub-or-Amazon-Echo-Show-interface-and-what-is-it-made-with

I know that The Google Nest Hub is running on Fuchsia OS and The Amazon Echo Show on Fire OS but what I want to know is what app they are running on there. Like what did they use to create the interface that you can naviagte through with touch? React? React Native? Native Android (Java/Kotlin)? Flutter? Linux program? Chromium?

And also how do I recreate an experience like that. Like I want to have the same experience as with a device like these, that you plug in and it boots into your app making it look like it’s just build for this purpose like these smart hubs, the Tesla dashboard or anything like that. Do I have to boot for example a Raspberry Pi in kiosk mode? And if yes, is there a native approach or some code/app I have to use?

PS: Yes, I have been googling a lot but I could only find the OS but not the software.

Found an answer after some more research. The interface was developed with Flutter.

By looking at the https://flutter.dev/ website, I recognized the google nest hub layout in the hero image.

Then I found another source:

Fuchsia is an open source project supported by Google aiming at
creating a new OS. The first device running Fuchsia (the Nest Hub) is
now in the wild, and the whole interface was implemented with Flutter.

https://blog.bam.tech/developer-news/how-to-run-flutter-apps-on-fuchsia