See the question and my original answer on StackOverflow

With WinUI 3, most namespaces usually start with Microsoft, instead of Windows (which was more for UWP).

It's actually difficult to get to the WinUI3-only documentation, here is some: Manage resources with MRT Core

The WinUI 3 resource entry point is now the Microsoft.Windows.ApplicationModel.Resources.ResourceManager class.

So, for a "MyString" string resource in a "Resources.resw" file

enter image description here

You can now do this:

Microsoft::Windows::ApplicationModel::Resources::ResourceManager rm{};
auto str = rm.MainResourceMap().GetValue(L"Resources/MyString").ValueAsString();

Note that you need to #include <winrt/Microsoft.Windows.ApplicationModel.Resources.h> after Xaml includes to compile.