How do I change the default country on the billing address form in Commerce 2? I want to set the default country when customers check out in Commerce 2.  How do I do that?
You need to edit the form display for the Customer Profile Type.

You can find it in the Admin menu under "Users", or go directly there by this path:

/admin/config/people/profiles/manage/customer/form-display

Then, on the widget for the address field, click the gear to set the default country.