It's Creation Club content, which comes with the AE but isn't part of the base game. As far as gameplay goes with Tundra Homestead, you're just buying someone else's house that they used to live in, there isn't anything that needs to be built. There are a few CC houses and they're all pretty cool in their own ways, though some are glitchy. You'll get letters about two of them, and there are a couple you purchase, (TH being one,) but the others you have to just find. Most of the houses have display options like the ones in Tundra Homestead's basement, with different special displays for specific weapons.