All 7 Layout Widgets (Row, Column, Stack, Container) questions
More in Layout Widgets (Row, Column, Stack, Container)
What are Row and Column, and how do they let you arrange several separate child widgets either horizontally or vertically alongside one another? What is the Container widget, and how does it let you conveniently combine padding, margin, background color, and border decoration all together within just one single genuinely convenient widget? How does the Stack widget let you genuinely layer several separate child widgets directly on top of one another, and how does the Positioned widget let you precisely control exactly where each individual layered child actually appears within that Stack? How do mainAxisAlignment and crossAxisAlignment properly control the specific alignment of children within a Row or Column along each of those particular widget's own two genuinely separate distinct axes? How does Flutter's own underlying constraint based layout system genuinely work, specifically regarding how constraints actually flow down the widget tree while sizes actually flow back up? How can you properly build a genuinely fully custom layout widget by directly extending MultiChildLayoutDelegate or CustomMultiChildLayout, for a genuinely specific complex arrangement not already covered by any of Flutter's own existing standard layout widgets?
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Flutter › Layout Widgets (Row, Column, Stack, Container)

How do Expanded and Flexible let you properly control exactly how much genuinely available space a specific given child should actually take up relative to its own siblings within a surrounding Row or Column?

Intermediate

Answer

Expanded forces its own specific child to genuinely fill absolutely all of the remaining genuinely available space left over within its own surrounding Row or Column after every other sibling has already been properly measured, while Flexible lets a specific given child genuinely grow to fill available space while still allowing it to remain smaller than that if its own actual intrinsic natural content genuinely does not actually need that full available amount of space, and both widgets accept an optional flex property specifically letting you precisely control the exact relative proportion of space that particular specific child should actually receive compared to its own other sibling widgets.
Row(
  children: [
    Expanded(flex: 2, child: Container(color: Colors.blue)),
    Expanded(flex: 1, child: Container(color: Colors.red)),
  ],
)
Real-world example A split screen layout uses two separate Expanded widgets with a flex ratio of two to one, correctly ensuring the left panel consistently takes up exactly twice as much genuinely available horizontal space as the considerably narrower right panel.

Common follow-ups: What is the specific practical difference between using Expanded and simply using Flexible with a fit set to tight?;What genuinely happens if you try to place an Expanded widget directly outside of a proper surrounding Row or Column?

Responsive & Adaptive UI Design;Flutter Widgets Fundamentals (Stateless & Stateful)
All 7 Layout Widgets (Row, Column, Stack, Container) questions