The engines themselves have gotten better at pushing pixels too.

Remember all the hype about Euclideon "infinite detail" stuff back in the early 2010s? How they had a data structure that pre-sorted their voxel data in such a way that they could switch between rendering big and tiny voxels depending on the player's point of view, seamlessly and in real time?

We have that now, just with polygons instead of voxels, which actually makes it even more technically impressive since Nanite has to maintain the mesh's coherence (though I guess in some ways Nanite is a bit worse, since there's only so much it can reduce a mesh before it disappears, whereas you can just keep making voxels bigger and bigger).

The foliage you see in that forest demo is Nanite geometry.