While emulation is popular, there is a purist method that uses actual PlayStation 2 hardware: .
For most people, It feels online, you can trash-talk over voice chat, and it just works.
To "play online," we have to trick the game, the emulator, and the network into thinking two (or more) PS2s are sitting on the same sofa.
Racing another human on Complex String or Seattle Circuit after 20 years of only racing AI is a transformative experience. You see braking points you never noticed. You learn defensive lines. The game becomes new again.