The "Aha!" moment occurs when it's revealed that a meteorite struck the Shibuya district of Tokyo. All the participants were victims of this disaster.
The Season 2 finale of Alice in Borderland "cracked" the series wide open by finally revealing the true nature of the games. Far from being a futuristic experiment or an alien invasion, the Borderlands were revealed to be a between life and death. The Real-World Revelation
Those who died in the games actually succumbed to their injuries from the meteorite strike in reality. Those who "cleared" the games were given a second chance at life.
After the final game, survivors were asked if they wanted to become "permanent residents" (citizens) or decline. Those who declined, like Arisu and Usagi, woke up in hospitals with no memory of the games, though they felt a lingering sense of connection. The Joker Card Explained