I've always been surprised that Apple added this functionality to the iOS home screen without having a solution to the reorder vs nesting UI problem. Trying to move an app into a folder often results in the folder deciding to fly out of the way and let the item take its place when you're really trying to drop something on the folder to insert it.

Android has the same problem. Sometimes it lets you drop inside, sometimes it flies away.

In IOS, I'm pretty sure you cannot add an app that currently sits on the far right of the screen to a folder anywhere beneath it.

Just tried this, and you can. But it definitely takes more finesse than any other column.