In the US, #postal General Delivery is a very important service to understand, and it's made very difficult to find information about. #Unhoused people and others without a residential address can receive #letters and #packages at a single specific post office in each city.
Figuring out which post office it is requires detective work: the only way I've found is to wrestle with the voicemail at 1-800-ASK-USPS until the system turns you over to a human, then ask for the General Delivery location for your specific city.
You need to complete a form at that post office to receive General Delivery mail. Then senders address things like this:

Recipient's Name
**General Delivery**
City, ST ZIP# of General Delivery post office.

Recipients collect their #mail from the post office by requesting it by name at the counter.