Hi,
The maine fields "email", "username", "password" are not custom fields as they have to be treated in a specific way, not like custom fields. For example, the password field follows the password restriction settings of the Joomla user manager settings area, and the email field has a specific format required and needs to be synchronized with the Joomla user account email address.
So if you need to customize them, you need to edit the view where they are displayed.
However, in your case, I wouldn't change them. I would simply change the "registration" setting of the HikaShop configuration to "simplified with password".
It will remove the username and name fields and the username will automatically be filled with the email address of the customer. That way, they don't have to think about having a username and it is transparent to them.