does anyone who works at a store like walmart know what they do with returns that have been opened?

Depending on the condition of the item, it gets shipped back to the wharehouse or manufacturer, or it gets reshelved (if there was no defect, etc...)

^^^ exactly, they repackage it, and resell it

return to manufacterurer for a new one.

This depends heavily on the store. For walmart, they will usually ship it away. For large electronic chains like Fry's electronic, they will repackage and sell the returned item as normal by placing a new item warranty on it.