With WoW they would obviously run each continent as an independent “instance”, but for Wrath, they clearly had multiple servers handling Northrend. There were places where mobs would not path over which was where the two “instances” would overlap.

WoW later introduced a kind of dynamic overlay of the same open world area from two realms where it was not very populated in the area so that players would be more likely to actually see other players.

There was also a feature where you could party up with a friend on a different realm and you would transparently be playing on their server in the open world (with restrictions like not being able to trade with them).

I haven’t played WoW since Mysts, so I don’t know how it’s changed since, but WoW was definitely more complicated than just sharding and instancing.