I have had a look through the forums for this answer and none of them suited. I don't want to use an app or hide my out of stock items. I just want them to appear at the end of the collections. Please advise.
Using debut theme.
please check our app Nada - https://apps.shopify.com/nada-sort-hide-out-of-stock
The app is automatically sorting collections in real-time, so the sold-out products are always at the end of the collection.
The easiest way to do this is on each collection page in administration, there is a way to sort the products in that collection. If you choose "price high to low" there, that is how it will show when someone is on your website, and it will automatically put the sold out items at the end of the collection.