I don't know how the author of Nessie specifically did it, but generally on the .NET platform, you have access to the Trident rendering engine by adding the WebBrowser control to one of the forms of your new application. From there, you gain all the event based functions and callable methods you'd expect a browser to use. I've written several of these type of 'toy' browsers over the years, and they work just like IE does. The only caveat is that you have to patch the registry first to allow the control to report (and render as) a useragent above IE 8. You can set it to report/render as IE 11 as the maximum.

Via NuGet, you can download the WebBrowser2 control which is based on the new Edge Chromium based renderer. I haven't seen any new browsers using this properly yet (and I've not played with it either yet) - it will be good when people do, because although the renderer will still be Chromium, the browser UI can then be anything you want. I'm looking forward to people challenging the accepted browser UIs that we currently have.

Of course, if you want to avoid the WebBrowser2 control, there's webkit.NET and Embedded Chromium which also can be built to bind with .NET apps.