How to Enter Fields in the Order Information form (and confirmation email)? I need to add additional fields in the customer's information form during checkout, such as Telephone number and "How did you hear about us?".  I don't see within the Commerce pull-downs in the Admin panel any way to do that similar to how fields are added in other Drupal forms.  I assume Commerce allows such customization.  Of course that data needs to be added into the database records as well.

In addition, how are those additional fields added to the email that is generated when the order is completed?  While the customer may not need it, other people within the ordering / fulfillment process are designated as getting copies of that email and need that information within the email.
Adding additional fields to the customer

You'll want to add the fields to the customer profile:

/admin/config/people/profiles/manage/customer/fields

Editing the email text

According to the Commerce documentation:

Use the template file located in /commerce/modules/order/templates/commerce-order-receipt.html.twig.