https://code.tutsplus.com/tutorials/how-to-create-custom-wordpress-writemeta-boxes--wp-20336
The WordPress post and page editor has built-in meta boxes for configuring things like categories, tags, and the featured image. If you want to let users add custom metadata, perhaps for a custom post type, you need to create custom meta boxes.
That's just what you'll learn how to do in this course. Follow along with Rachel McCollin and learn how to code a plugin that creates custom meta boxes in your WordPress admin screens.