Answer
Combine several techniques: use star-sizing Grids and FlexLayout for fluid resizing; use OnIdiom and OnPlatform to vary values by device type; use VisualStateManager with size triggers or the DeviceDisplay metrics; and use the AdaptiveTrigger-like state triggers to restructure UI at breakpoints for phone vs tablet vs desktop.
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="{OnIdiom Phone='*', Desktop='300'}" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
</Grid>Real-world example A master-detail app shows a single stacked column on phones but a fixed 300px sidebar plus content on desktop, driven by OnIdiom.