Sorry I didn't use any collision for this game but you should check Tilemap Collider for collision on isometric map. Unity also have very good tutorial on Youtube on this topic.
hey, sorry for replying so late I got busy due to my other game.
So for falling mechanics I just check it that tile has any platform sprite in it. You can get TileData for any tile easily if you have the coordinates of the tile. If there are no tile simply kill your character.
