Out Of Stock auto-unpublishes products when inventory reaches 0, hiding them from collection pages, search results, etc to improve the shopping experience and increase conversions. When inventory is added again the product is republished, automatically!
Note: you cannot hide variants from collection pages. It is done on a product level.
Note: auto-hiding and auto-unhiding of products only applies for products from the point of app install. There is also a manual hide and unhide functionality that can be triggered from within the app to hide and unhide products.