How does The Final Horizon DLC play out?

[AngeryBlueBerry]

I just beaten the game earlier and started the Final Horizon DLC. This looks like an alternative timeline to when I first appeared on the last island I think and I'm playing as Amy and need to uncover the island again. Are all the islands available to play on or just Ouranos? And do I have to uncover the map 3 times with each of the character or does the map's progress carry over with the other playable characters? Meaning if I complete a challenge I don't need to do it again with Knuckles or Tails and vice-versa. I'm just confused how this all plays out

[Mez Koo]

Its one map for all 4 characters. You can use mods to play as any of the characters throughout the whole game.