WooCommerce SQL Tables

I want to add custom field in a WooCommerce Billing Details page form.

Does anyone offer a video tutorial to learn how to add two custom fields and delete one existing field in the Billing Details form.

Also I want to pass those changes on to show up in the order email and in the pending order Admin area, and I am working with a Child Theme so I do not want to make any changes that get deleted when I update the plugin going forward.

I need to build a COUNTY field with a drop-down to display 30 counties, and eliminate the drop down for the STATE field and make it just one static STATE with no other state choices. I need to receive those custom fields in the order email as well.

Any help would be greatly appreciated either by video tutorial or the documentation to show how to perform those tasks.