Using available plugins saves you a ton of time and effort, but some like to develop their own as a learning opportunity and/or they are very particular about how they want something to work. In that case, start with the Plugin Handbook. The menus section tells you how to add your own pages to the admin area. If you want to add dashboard widgets, they are not actually widgets, they are a special kind of metabox. Review /wp-admin/includes/dashboard.php for examples.
Even if you do not intend to use existing plugins, they can be a useful resource in guiding you on how to accomplish certain tasks. Plugins at www.ads-software.com are GPL licensed, so you can use portions for your own purposes. Just be sure to attribute anything you use. If you are learning, you should really write your own, but it’s still worth studying what others have done.
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