https://www.linkedin.com/learning/using-custom-meta-boxes-in-wordpress

When you're building your own plugin or theme, it's important to reduce the number of external dependencies that a site owner needs to use it. While there are a lot of great custom field plugins out there, CMB2 allows you to include it as a library in your own theme or plugin. 

In this course, Joe Casabona, course developer and founder of Creator Courses, walks you through how CMB2 works, when to use it, and how to build out a website using it within a custom plugin. Joe goes over defining custom fields, creating meta boxes, adding a custom field type, and adding fields to your meta boxes. He shows you how to add a file uploader, create a repeatable field, get data from CMB2, and much more. After covering how to build a post select box, Joe explores some noteworthy features, such as frontend meta box support, creating a theme options page, and adding fields to user profiles.