To integrate Flutter code into an existing iOS app, 
  check out Add Flutter to existing app.

This document can be used as a cookbook by jumping around
and finding questions that are most relevant to your needs.

Overview

Views vs. Widgets

How is react-style, or declarative,
  programming different from the
  traditional imperative style?
  For a comparison, see Introduction to declarative UI.

In UIKit, most of what you create in the UI is done using view objects,
which are instances of the UIView class.
These can act as containers for other UIView classes,
which form your layout.

In Flutter, the rough equivalent to a UIView is a Widget.
Widgets don’t map exactly to iOS views,
but while you’re getting acquainted with how Flutter works
you can think of them as “the way you declare and construct UI”.