There isn't a standard way to do this, and all depends on people using random "Geo IP" databases. Most of these databases are made using a variety of sources, t

There isn't a standard way to do this, and all depends on people using random "Geo IP" databases. Most of these databases are made using a variety of sources, though primarily the easiest is to see what ASN the IP address belongs to, and then you just put all those IPs as belonging to that ISPs country. This covers 90% of the domestic users. What ExpressVPN has to do is, simply convince one of these ASNs to allow announcing a IP block from them on BGP, and that will work until some GeoIP database finds out and changes the country for whatever IP block ExpressVPN got. In some cases, traceroutes from different locations around the world can be used to roughly triangulate the location. Though you may find the datacenter to be inside of earth if the IP is actively being used for unicast :)