This is a fun question. Going purely on my experience of playing the Sims like ten years ago and never since, here are my suppositions:
Obviously Niall meticulously builds his houses from scratch, carefully laying down and then moving walls until they suit his vision, tastefully decorating each room, giving good thought to the functionality of the space for his little computer friends. “The sink and the stove and the refrigerator should form a triangle in the kitchen for maximum efficiency,” he probably murmurs to himself. “But which fridge?” When he’s done making one house he just starts building another one, neglecting to actually play any of the characters.
You are right to assume that Louis is the monster who removes all the doors and toilets and then gleefully waits for his Sims to pee and catch on fire and die, but also once in a while he drunkenly feels bad (and a little spooked) about it when their ghosts come back to haunt his next victim. Occasionally he gets mad that you can’t just cut out the middle man and play as the Grim Reaper. Gets distracted and quits after he runs out of ways to cause mayhem, because there are only so many times you can take the ladder out of the pool while someone is swimming in it and have it still be fun.
Liam has absolutely no idea how to cheat, motherlode / rosebud codes included, and spends the whole time frustrated because he’s playing honestly and as a result his Sims have to like, diligently go to work all day, and he can’t buy the nice food, and how the hell did Niall afford to make his house look so nice?? All his Sims are always like, moderately happy, and usually don’t die or hate their lives or anything, but he always feels vaguely guilty that he can’t make them all achieve all of their goals. Probably plays a preloaded family.
Harry just makes infinite identical clones of himself in the character designer and then makes them whoopee with each other a million times in a room 100% decorated with leopard print. “Nice,” he tells them afterward. “You guys looked really great in there.” Probably has that patch where you can see their weiners.