The Retron5 is a game console created by Hyperkin specifically to emulate older systems, primarily by using the original cartridges. While various official controllers can be used from the emulated consoles, the Retron5 is equipped with its own wireless controller. To play the games, the Retron5 utilizes RetroArch.
Shortly after the release of the Retron5, it was discovered that Hyperkin were using RetroArch, which, due to the fact no source code was released for the Retron5, meant that Hyperkin were abusing the license. While they did later acknowledge this, it was found that not all of the source code was provided. This has yet to be resolved.
The source code for the libretro cores used can be found here.