8 x 8 km playable area

Okay Playable area, now why is playable area is only limited to 8 by 8 km, then how then does Horizon Zero dawn, goes 20 by 20 kms or greater than that, is it world compasitions or do they create multiple 8k by 8k grids then join then snap together. also I have notice you know that green grid everyone creates and sculpt Mountains on just below that their is another grid bellow black gridlines on white goes on endlessly I gave it a go its never ending so if that goes on for enternity, So why is the grid sculpt on only limited, is there a way to expand beyond 8 by 8 km to 50 km by 50 kms?

There’s a limit due to floating point precision, there’s a limited amount of memory that a number can take up so when a number gets very large it loses more decimal points and the steps between numbers is higher, so movement is no longer smooth, and then things like collision can’t work because it can’t actually detect the point where something hits accurately enough.
That’s why there’s world composition, the engine has the world split up into pieces and as you move between pieces it shifts everything so that it’s back at the 0,0,0 origin and you don’t reach the point where you lose precision. I think the point where you loose precision is actually like 6km x 6km though.
For Horizon, that’s not done with UE4 so they’re not using the tools in UE4 but the concepts are the same, they would have split up the world into pieces and load them in when needed.