In case you’re just now looking to start using custom fields, you’ll enjoy the displaying WordPress custom fields tutorial on wp-types.com.
It’s mainly geared towards custom fields created using Types plugin, using either Types PHP API, or Views. However, there are also two lengthy appendices that dive into the internals of how WordPress stores custom fields in the postmeta table and how to access them in different methods.
I think that this tutorial can be pretty useful for developers with different technical skills. Obviously, if you’re just starting your WordPress journey, this will let you cover all the basics, so you’ll be able to build full sites. If you’re already using custom fields, the appendices can add insight on how it’s built.