The required file is the (a .PUP file) downloaded directly from Sony’s website. Once you point RPCS3 to it, the emulator installs it like a real PS3 would.
When you install the firmware into RPCS3, the emulator extracts the necessary system modules—such as the libraries required to render graphics and process audio—so that your games can interact with a virtualized version of the PS3 environment. Where to Download Official PS3 Firmware firmware rpcs3
The RPCS3 developers have streamlined this process significantly. The required file is the (a