Short and simple: Yes, you can access the native UI even if there is not a plugin.
Those plugins are to provide some helpers so yo don't need to type everything for both platforms on your own.
For example, if you want to create a label on an Android platform, without using the plugin, is as easy as:
new android.widget.TextView(this._context);
You can take a look at all the UI components in their GitHub to see how the plugins work in both platforms:
https://github.com/NativeScript/NativeScript/tree/master/tns-core-modules/ui