When you add a custom field in checkout page it gets created either as Order Field (order meta data) or as User Field (user meta data) based on the settings.
Order Fields (Order Meta Data)
Order fields will be saved in wp_postmeta table as Order Meta data and will be displayed in multiple places. To create Order fields you must select the checkbox “Order Meta Data”
User Fields (User Meta Data)
User fields will be saved in wp_usermeta table as User Meta data and will be displayed in multiple places. Custom user fields values can be updated from user profile page. To create User fields you must select the checkbox “User Meta Data”
When you add an custom checkout field it gets displayed in different pages and emails based on the selections you made in field settings using the check-boxes marked in below screenshot,
The following screenshots shows the different places where you can see the custom fields and the input provided by the customer.