Geolocation fields are not custom fields on Accounts/Contacts, but those are compound fields and are part of the Address field. You can though create custom Geolocation fields on custom objects to store latitude and longitude values.
So to access the fields say on Mailing Address or Billing Address, you will need to access it using their individual API names as below:
MailingLatitude, MailingLongitude, BillingLatitude and BillingLongitude.
For your first question:
Can anyone confirm how they are populated?
These fields are automatically populated, if you have Data Integration Rules activated in your org. The documentation does not mention if you specifically need a separate data.com license for this, it just mentions the editions with which this service is available.
Activate Data Integration Rules
Activate Lightning Data package rules, geocode rules, and company info rules.
Available in: Professional, Enterprise, Performance, and Unlimited Editions
As for your other question:
and does it cover addresses globally or only a specific region?
Yes, the fields capture the latitude and longitude based on the address. That signifies that its not restricted to any particular region but works as long as the address entered is accurate.