The equivalent to Android’s density buckets are:

ldpi

0.75x

mdpi

1.0x

hdpi

1.5x

xhdpi

2.0x

xxhdpi

3.0x

xxxhdpi

4.0x

Assets are located in any arbitrary folder—Flutter has no
predefined folder structure. You declare the assets (with location) in
the pubspec.yaml file, and Flutter picks them up.

Note that before Flutter 1.0 beta 2, assets defined in Flutter were not
accessible from the native side, and vice versa, native assets and resources
weren’t available to Flutter, as they lived in separate folders.