The USB sled—the USB external storage docking station, etc—connects to the USB bus, just as does an external hard disk drive you've seen and probably used. The difference with the sled is that the hard disk is a whole lot easier to get at. With the sled, you don't need to disassemble the USB device to get at the hard disk.
The USB sled here would preferably have its own separate power supply, and would not draw power from the USB.
The sled powers the contained SAS/SATA device, or whatever other sort of storage device that the sled was designed to hold.
Hard disk drives can over-draw the available USB power when connected to USB-A connectors and USB-prior-to-USB4 buses.
USB-C / USB4 / USB Power Delivery can have vastly more power available than does a USB-A connector and USB-prior-to-USB4.
But again, if your original Fusion drive was flaky due to a failing hard disk drive, there's a reasonable chance that this configuration won't work.