How to Add Existing Fields to a Custom Form

We had a custom user registration form for use in our Commerce checkout account registration workflow that allows users to register during the checkout process. We wanted to add additional fields to the custom registration form but adding the fields to the standard user entity would not display the fields in our custom form as its fields are all created manually in the custom form.

In Drupal 7 we would use the field_attach_form() function but it is removed in Drupal 8. Below is how we achieved this in Drupal 8/9.