Playstation also had a trapdoor Parallel I/O port exposing raw address/data bus, it was meant for network interface, debugging(PSY-Q) and stuff(ActionReplay/GameShark). Great thing about that port is you can hang your own ROM there and console will execute it while booting, no code signing/drm crap.
Afair at the beginning PSIO patched original firmware replacing all CD routines with its own, but later in the project it was discovered a lot of games talked straight to the hardware ignoring SONY requirements for using BIOS routines. This is why current version comes with small board you need to solder inside to reroute chip select signals from the CD controller chip - PSIO emulates that chip completely. You still get data faster than CD due to no seek times. https://www.youtube.com/watch?v=Wc3rOb7Evxc
The problem with a lot of the modchips is that the companies behind them are secretive (leading to loss of knowledge when they close) and they're just so damn expensive.