Is XAML in WPF equivalent of .Designer.cs in Windows Forms apps?
Does it just provide compile-time state for the UI?
I am not sure but it looks like you can do things programmatically with XAML at run-time.
If I have a basic UI state where everything is added at run-time, then should I be looking outside the XAML stuff?
It’s probably safe to look at XAML that way – although it’s not entirely accurate. The XAML is compiled into BAML, and parsed at runtime – where the Windows Forms designer.cs file is just another C# file built by the designer. XAML is never directly translated into C#.
You can do everything done in XAML via code, though. Charles Petzold’s WPF book actually takes this approach. It builds entire WPF applications in code before he ever introduces XAML.