When you add a widget to your master page, the widget is not automatically added in the other master pages which inherits from the main master page, because the widget that you have added is saved in the database and is not applied to the ASP.NET master file. If you add your code manually in the main master page in Visual Studio then you will be able to see the changes.
In order not to add the widgets to all of the master pages, when you create them you can base the new template to you custom master page. For your convenience I have recorded a sample video demonstration
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