Well, Phantom Liberty in and of itself is a pretty big expansion with a whole new area to explore, a bunch of new vehicles and weapons, a new ending, a really big add on with sidequests and a big main quest.
But more importantly is that it came alongside a really huge patch that literally changes everything. Like, it changes clothing so it's no longer how you get your buffs. With few exceptions, clothing is no longer how you get buffs. It refined car combat. It completely overhauled core mechanics like how your hacking works, how much of your body you can modify with what kind of quality upgrades depending on your stats. The police AI is completely different.
Like, not so much Phantom Liberty in and of itself, but the patches that came alongside it literally changed the game. You take vanilla Cyberpunk from its original release and compare it to what it is now, it really does feel like a completely different game.