The customers who left the devices at Apple stores for repair will not get them back until the stores reopen.

On March 14, Apple officially closed every one of its stores outside of China over coronavirus concerns — but stayed open for two days after that so that customers could pick up iPhones or other devices that had been left for repairs.
The report also added, “Apple sent emails and tried to call customers who had devices being repaired at its stores, but some customers couldn’t be reached, or otherwise couldn’t make the two-day pick-up window to get their devices.”
A spokesperson said, “There certainly are people that, for whatever reason, did not pick up their products before we closed and their products are at our stores” he also added, “We made every possible attempt to get people’s products back to them.”
These customers who did not pick up the devices will not have to wait until the stores reopen. Apple’s original timeline to open the stores was March 27, but now there is no telling when they will open. It could take weeks or even months before the stores reopen.
As of now, Apple is also trying hard and working with the customers to send the devices back to their homes rather than the retail stores to pick up.