Difference's in areas to og UT
Ruins -> Extermination Zone
The flowers you originally landed on is now just a huge pile of corpses, some being alive actually, and violent, asgore, and some other monsters there have made it they're duty to kill those undead so they don't go into the rest of the underground, these undead don't exactly cause an infection but they are still very dangerous, the ruins now exist as a partial safe place but mostly an area where they exterminate undead humans.
What was once a nice snowy area, is now a stone cold Toxic gas filled danger zone, it's required that you stay in doors and don't go out, inhaling the gas harms most anyone, even those who don't have lungs, best to go out if you have a gas mask on hand, there are also still many traps laid around, ment to take out the undead if they got threw, still can also harm normal people and monsters as well, so watch your step.
Waterfall -> Drought or Waterlessfalls
Waterfall has lost all of the water and is now a dry land, some water hidden away but not alot, alot of the sea based monsters have ether died off or been taken to an aquarium now, but with the water gone, some secret areas have opened up that monsters hide away in, or just hide stuff in, there also seems to be a monster hunting others in there when they enter, locals have nicknamed this monster the hunter, also due to being close to staydin, there is some slight toxic gas there as well but not alot, gas mask still required.
Kind of self explanatory, hotland is now cold and frozen over, lave is now stone and the core has broke down, not much toxic gas here, but now you have a chance of just, freezing up, lot of the steam there now just ether doesn't work or blows out cold air, and also most the puzzles and traps don't work, and certain areas are blocked off so shortcuts are required to figure out where to need to go, (...shortcuts are required- that doesn't make sens- ah whatever)
MTT Resort -> Resort Bunker
Mtt resort is now just a place to keep people safe and healthy, mtt works there but the place isn't named after them, kinda simple but yea, still pretty cold tho-
New Home is now a part of the underground that has no issues like the rest of the underground does, it's nice, calm, and completely safe, where you can live your life with no worries or problems, a wall is in progress of being built around it currently to keep the citizens within it safe and to get other monsters into it as well
The place the nukes first hit, an old kingdom, near the ruins/extermination zone, a place once breathing with life, has now been taken over by plants and other wild shrubbery, and alot of golden yellows flowers, the way into it has been sealed off, with no way back in, I mean there probably is, but it's gonna be hard to do so.