It's a particular module for the Xilinx Artix 7 XC7A100T, made by a Shanghai-based integrator. As an individual user, you could buy these on Aliexpress, from eBay resellers, or from Amazon resellers. Facebook presumably has an arrangement with the contract manufacturer that's not going to be possible for an individual developer ordering ones and twos to replicate.
However, to modify the HDL, you'd need a $3,000 license for Xilinx Vivado, which is pretty much a nonstarter for amateurs.
However, to modify the HDL, you'd need a $3,000 license for Xilinx Vivado, which is pretty much a nonstarter for amateurs.
[1]: https://store.digilentinc.com/arty-a7-artix-7-fpga-developme...