A few of the Google apps are. Google Pay (the new one), Google AdWords, and Google Classroom are big ones. Stadia and Grasshopper were both done with Flutter but are gone now. Toyota, BMW, eBay, Tonal Fitness, HERE Maps are all using Flutter.
I have exactly none of those installed. The only Google app that I have installed on my phone is Meet, and I wish I didn’t have to have that installed because it’s the worst meeting app on my phone, but the company uses Google Workspaces.
I will go without an app rather than a Flutter app.
Which big apps are written in Flutter?