Hello.
Nested pages was a feature available for Portal engine (WebForms)-based websites only. In Kentico Xperience 13 and .NET Core/MVC5-based sites, you will need to utilize page layouts, view components and sections, depending on your exact use case, as per the official MS doc: https://learn.microsoft.com/en-us/aspnet/core/mvc/views/layout
Kentico Support