I love the Matt Risinger Build Show on YouTube. He’s done many episodes based off of his own personal home build in Texas and he has an AMAZING(!!) conditioned attic that is completely livable. He’s a total building sciences nerd and he did an entire episode on the insulation he used inside his home (RockWool mostly). Fun to watch.
Today . . . . on the Build Show. Some of his stuff is a little over the top, but the majority of it is best practices. There is a cost benefit trade off, and on new construction it works, but tying to retrofit existing structures sometimes does not pay off. If I was building a new house, I would use many of his methods.
Most attics here in the Northeast are not conditioned space. It can be done, and it is not unheard of, but more often than not it is insulated on the attic floor.
Many times, especially in older homes, the air handlers are in the attic, since central air was added after the fact, and we have a lot of steam or hydronic heat here. There are forced air homes, and those of course can be upgraded to central air easily. But when you have to cut in new duct work, the attic is a lot easier to work from.
We also get the swings in temperature, so my attic goes from really cold to roasting day by day.
Basements are hit or miss around here. Some houses have them, others do not. It is a style and water table thing.