Is it possible to reserve or hold a product if it's unavailable?
Im curious if it is possible to reserve or hold a product from a store if the store does not have it available, or if stores are unable to hold a product that way
Unless its a discontinued product, its highly unlikely the specific product or device you want from an Apple Store would not be available. They generally keep stock of most things in store at all times, unless its an odd configuration for things like Macs. In which case you will be allowed to order it, and the store should provide an estimated date of arrival to pick it up.
For things like pre-orders, you do so before they become avaialble, and are given a release date and in-store pickup availability date during checkout.