I guess you can just start loading a first batch, add an intersection observer to the last 3 elements (if you have 3 lanes) and then when one of those intersects you simply start fetching the next.

I suppose just checking scroll height of the container? Once you're x pixels above the bottom, fetch more. Not the smoothest, but doable

You just append new <figure> elements to the <main> in the example and it will automatically put them in the appropriate column.