Do expansion packs replace the base game?

pets is your newest installed game it combines with the sims 3 base game (the one you had first )for playing the game always plays from the newest installed released expansion or stuff pack lets say for example you get supernatural as that would be the newest installed game that will combine with sims3 and pets and you play from that one BUT lets say for example later on you buy  generations that too will combine with the others but you will still play from supers because generation is older than the other games and supers is the newest installed released game the game always plays from the newest installed released game

any world that comes with a pack will be in a drop down menu say for example at the moment you have two worlds sunset valley the base game one and appaloosa plains the pets one you can select either world to play in in the main menu of the game is a +new game if you click on that a drop down menu will show up and you can select either world to play in if you choose to play in sunset valley and as you have pets animals will show up in sunset valley