EDIT: Here's an explanation I saw earlier of how it's done in Finland:

How we do it here in Finland is that there are digital identity providers which use bank/mobile carrier to identify you. They then use MFA when identifying you. Any service can use these services to do strong authentication for you. And they don’t cost anything for the customer, and is really cheap for the company who wants to identify you. It is also build into the law that you must identify people using these, to avoid identity theft.