> How does it work exactly? Do you enter your number when you first open a web browser or is it a per website thing?

It's a per website thing, or at least it was ~10 years ago.

This lead to something -INCREDIBLY- stupid.

South Korea is where a -LOT- of MMOs start.

So... there became a market for Korean SSNs, purely so you can play video games.

I am 100% serious. The leading reason for using someone else's identity in South Korea is to play video games.