You would use the settings extension (it’s built in I believe) for that! (Or better settings, if you want more control.)
It will save when you reset and when you close the tab! Whenever you want to save the game, the settings you define will stay, so you can load them back into the game when it restarts. To load the save file in, add an if statement on start to check if the setting exists (if it doesn’t exist, it means the player has never made a save file, so you should load them at the beginning of the game) then if they do exist, load the correct tile maps/ position / stats / sprites corresponding to their save data.
Here’s how I’d recommend saving an undertale file:
- Save the player’s ‘level’ (the tile map they are on or the stage they are at in the game) as a number, rhen load the scene corresponding to that number when the game starts
- Save any stats the player has (idk like money? I’ve never played UT) as a number to load in
- Save the players position as numbers, then set the player back to that position when it loads
Let me know if you have may questions or want more specific help! 