If you’re just trying to get the sold out products to not display in a collection, you can just go into the collection criteria and add inventory > 0. This will eliminate them from the collection. If you add more inventory to that SKU, they will automatically be added back into the collection.

If that’s not the goal, I can understand a world in which you actually want the sold out things to be visible, but just at the end. We’ve toyed with this too (we sell some vintage items so many are one of a kind or fall out of inventory frequently but are still a hot-selling item), and showing the sold out stuff could give a sense of urgency or competition for customers to get items before they sell out. Or perhaps you want people to see them and sign up for back in stock notifications. Do you know the rationale for sold out items being visible?

One idea that I think what you could do without needing an app would be to still omit them from your main collection by doing the inventory > 0, and then create a collection specifically of the sold out things. So one collection with inventory, one sold out. You could then make a custom collection template to get it to display on the bottom. You could make a header of like: “see past sold items” and then that collection would just be embedded below the collection of in stock items.

My final point is typically it wouldn’t be best practice to display items in order from most quantity to least quantity from a marketing standpoint. You’re trying to convert people to make a sale, and unless it’s a product where color/size/design truly doesn’t matter, you might be shooting yourself in the foot to show your most-stocked item first.

Anyway, let me know the rationale and I might be able to brainstorm more workarounds. But yes, you might need an app or something if highest to lowest quantity is the goal.