To get a better understanding how the basics of how riverpod works I recommend taking a quick read of the documentation: https://riverpod.dev/ or take a look at one of my older video of riverpod. That video is a bit outdated codewise but the core concepts still apply and should help you get started!
Why did I pick Riverpod and MVU architecture?
First let's just touch on Riverpod and previously I was quite heavily using Provider though this had a huge drawback that I personally didn't like at all, which was not having proper compile safety. The more a project grows the bigger the issue this becomes. Getting a ProviderNotFoundException was not something I liked to deal with and with Riverpod that issue doesn't happen as one of the main features of Riverpod is being compile safe. Now the reason I also picked StateNotifier compared to let's say ChangeNotifier or ValueNotifier is that StateNotifier is that it promotes data being immutable.
Immutability is an important concept that directly ties in the architecture I use which is MVU. The architecture in itself is rather simple. You have the UI and you tie it to something like a StateNotifier. With this, you won't have additional layers that don't provide anything. MVVM is another architecture a lot of people like to use but personally I don't think it fits the paradigm of how Flutter works as we don't need a "binder" to update the UI as we do with another tech that for example uses XAML as UI.
That said, architecture is just a guide to help you write better and more structured code. There is nothing wrong with taking parts of an architecture