Where to manage customer metafields, aka custom fields:

Once you've created a customer field, it can be managed by simply opening any customer account through the Shopify platform here:

Once opened, you'll see the new 'Metafields' section added to the customer with the custom field you've created.

These fields can be populated either by the customer on the customer account page or, as a merchant, you're also able to populate these fields.

Not sure how to add fields to the customers account page? Check the help guides below