How can I implement a custom 404 page in ASP.NET Core?

You can create a custom 404 view (e.g., `Error404.cshtml`) and configure your `Startup.cs` to handle `NotFound()` results. Use `app.UseStatusCodePages(builder => builder.UseRazorPage());` to automatically render Razor Pages for status codes. Alternatively, you can implement a middleware to catch `NotFound` exceptions and return your custom view.