How does my server (or user agent?) look up static files?

I am just wondering, how does it exactly work to retrieve a saved image on filesystem that belongs to some json object? E.g When I’m doing a product creation that has an image and I upload the image together with the newly created product, I usually save the image on my server backend in some folder on the file system and then save the returned image save path and store it as a string field in my product json object.