The first step: Convert json data to viewmodel.

You can use Newtonsoft.Json to deserialize the json data and assign the value into viewmodel.

The Second step: Add ShellContent programmatically with the data.

Assuming that you've got the viewmodel , it should be a List , e,g List<Model> Models .

foreach(var model in Models)
{
    ShellContent content = new ShellContent();
    content.Title = model.Title;
    content.FlyoutIcon = model.FlyoutIcon;
    content.Content = new MainPage();

    menu.Items.Add(content);   //menu is the name you defined on FlyoutItem in xaml
}

You can refer to docs about Shell Flyout. I found three solutions from the docs:

1. Try Binding the Icon to the ViewModel Property. The ViewModel Should be set to Binding Context of the ShellPage.
2. Customize the appearance of FlyoutItem by setting the Shell.ItemTemplate attached property to a DataTemplate:

 <Shell.ItemTemplate>
 <DataTemplate>
 <Grid ColumnDefinitions="0.2*,0.8*">
 <Image Source="{Binding FlyoutIcon}"
   Margin="5" HeightRequest="45" />
  <Label Grid.Column="1" Text="{Binding Title}"
     FontAttributes="Italic"
   VerticalTextAlignment="Center" />
    </Grid>
    </DataTemplate>
   </Shell.ItemTemplate>

3. Replace flyout content with own content by setting the Shell.FlyoutContent bindable property to an object:

 <Shell.FlyoutContent>
<CollectionView BindingContext="{x:Reference
shell}"    IsGrouped="True" ItemsSource="{Binding FlyoutItems}"> 
<CollectionView.ItemTemplate> 
<DataTemplate>  
<Label Text="{Binding
Title}" TextColor="White" FontSize="18" /> 
</DataTemplate> 
</CollectionView.ItemTemplate>  
</CollectionView> 
</Shell.FlyoutContent>