PCSX ReARmed is a fork (when someone takes a copy of source code and begins their own independent work on it to create something separate) of PCSX-Reloaded, a open-source console emulator that is the main version of PCSX from 2000.
Emulators are basically software made to run games made for a specific platform on a different platform, particularly on the PC.
"Classic" consoles, tiny, re-packaged versions of 80s and 90s game machines, are a popular trend in gaming. There's another bonus here, too: with the PlayStation Classic running open-source emulator software, it should be relatively easy to add extra titles beyond its 20-game library if you feel like some after-purchase modding. Oddly enough, these close-ups shots make it hard to remember that the PlayStation Classic is 80% smaller than the original PlayStation, in terms of volume, and 45% smaller, when it comes to its width and length. However, even those devices run on a custom Linux build, which is also open-source.
So yeah, before complaining you console gamers about PC gamers pirating the console games via emulators, know that the mini console you are about to buy is actually using the very same emulator you have criticised.