As a former store employee I can speak to this. Your technician knows nothing about you and your phone until you check in for your appointment. The online appointment system doesn’t sync with store inventory for safety and function purposes. More on reddit.com

No. But you’ll end up waiting a hour or two. If you don’t. More on reddit.com

Apple stores have most parts on hand, and same with replacements. If needed part orders take a couple days to arrive (customer phones are not held during this situation). Replacements are often in stock, same process for the order if out of stock. The repairs in an Apple Store are typically completed fully in under 70 minutes (depending on repair and store). Most stores have same day appointment availability depending on time of day with walkin appointments (book a heads may show full but walkins are still available with a wait) Apple phone support can try and book only book ahead appointments, and cannot see inventory levels in stores or reserve parts as everything is first come first serve from appointments to parts and service timing On rare occasions Apple Stores send devices to a depot facility (common with Mac’s rare with phones) due to part availability or engineering requests/discrepencies The appointment system is set up to allow both book ahead and walkin appointments and it takes into account the amount of scheduled technicians who are certified to take the appointment and the repair system schedules the repairs depending on how many technicians are scheduled to complete repairs during that time. Source: I have a fairly intimate knowledge of Apple Store workings and systems Edit: Mac’s are always a multi day repair requiring typically 3+ days unless software related issues only More on reddit.com

If you drop in early in the day, you should be able to see a technician same day. Just remember that they may have to order parts if they are not available in store. More on reddit.com