You can find commercial USB sticks that can also connect via WiFi. But [Neutrino] made his own using an ESP8266 married to a card reader. It all starts with the old trick of soldering a header to an SD card adapter. The USB port is still there, but it is only for power. A 3.3 V regulator and an ESP12E board round out the hardware.
Of course, the trick is the software. Starting from a few examples, he wound up providing an FTP server that you can connect to and send or receive files using that protocol.
It sounds like a few design compromises led to the device being somewhat slow, although it looked usable. [Neutrino] wants to change to the ESP32 and make some other changes to get better speeds.
Of course, you could go buy a SanDisk Connect Wireless or similar product, but what fun is that? Besides, being able to connect an SD card to your project opens up a lot of possibilities for data logging, configuration, and more.
Honestly, we’ve had relatively poor results with most of the commercial wireless memory offerings including the venerable FlashAir, so it is attractive to have a device that you can completely control. If you have an SD card connected to a project, one thing you can do is play some tunes.