Also, Flutter's install instructions say: "Flutter requires Android Studio to be installed and configured" .
The Flutter SDK hides this away nicely, but Gradle runs when you launch a Flutter app during development (on Android).
First there were SDK and NDK links.
Then there where SDK Tools that autoupdate and downloads the SDK.
Now there is only links to studio, an IDE that contains the auto update tools that downloads the SDK.
But if you want to jump all the hops, you might be able to download just the SDK. For now some old convenient links might still be up.
Similarly, if you want to run on iOS you need a Mac with xCode installed.
(It's rather weird to me that you need to install entire other IDE's as developer dependencies to build mobile apps, but that seems to be how it is.)
I don't remember the details, but a couple years ago there was a way to install just the Xcode build tools without installing the entire IDE. I'm sure there's a similar process for the Android SDK. (Though, of course, none of that is officially supported, so you're on your own for the most part.)
Edit: Turns out Google provides downloads for just the SDK without Android Studio, so it looks like the separation is unofficial only for Xcode.