To use your custom twig templates, first add them in some php file, like functions.php like this:
function addMyTemplates($arrMyTemplates){
$arrMyTemplates["mytemplate"] = "your template content";
return($arrMyTemplates);
}
add_filter("ue_get_twig_templates","addMyTemplates");
Then, inside the widget editor you can render the template like this:
{{ucfunc("render","mytemplate)}}
All the fields available in the widget data and items data will be available in the template as well.