How do you actually play multiple Sims and multiple households in this game?

As others have noted you get used to focusing on one sim at a time. When all sims in a family are home you can get them all to do different things, but sometimes you just want to take one sim out somewhere on their own, as a break and mostly the others just manage OK to take care of themselves. I have many saves and do rotate but find 3 families per save is my max, as otherwise it's hard to achieve goals because I keep aging on, even though use long life.