How to author controls as separate ResourceDictionary/Control Codebehind files

[Zulu-Inuoe]

I notice this project often makes use of the pattern of having eg RangeSelector.xaml and RangeSelector.cs, which the xaml being a ResourceDicttionary with a control template.

I'd like to follow this same pattern in my project, but I feel like I'm missing something - Where in this project do you merge all the individual ResourceDictionary definitions? In a standard application I'd expect to see this in AppMain.xaml, for example
Is there something else going on I'm not seeing?

Also, how is it that when I bundle this library in my project, that I don't have to import any ResourceDictionary like I do with WinUI like <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" /> ?

I appreciate any help. Thanks!