I have three ‘pages’: one with a selection of icons which I managed to group into folders (but forgot how…); another with ‘all apps’ (and one in between with the recently used apps).
How can I move an icon from ‘all apps’ in to a folder on the other page?
If you drop one app onto another, automatically, a folder is created - containing both apps.
This is a simplified scenario - I don’t know whether it really covers your case, though.
No matter, whether the app is inside a folder or not: you can tap the app long enought to get it hovering. Then, you can drag it around.
If you now drag it to the border of your screen, really the very border (left or right), the page will change while you’re still dragging the icon. Then, you can just drop it there, or drop it into an existing folder or drop it onto another app to create a folder.
At least I tried with mine just a minute ago and it seems to be working that way.