You can brood in the garage, it’s much better than in the house from a dust, noise, and possible smell standpoint. The more isolated they are the less likely you are to handle them.
This is my brooder built into the coop. It has a plywood top that is a droppings board but the bottom and sides are hardware cloth. In the summer it’s kept pretty open but in the winter when it can be below freezing the sides are wrapped pretty tightly with plastic. I drape the plastic to the ground to stop drafts and put a “floor” in the heated end to help keep the heat in. I have used a piece of plywood but wound up with a plastic floor tray for a dog crate and use that now. I only heat one end and don’t put a floor in the far end. Some mornings there is ice in the far end but the heated end stays toasty. There is enough ventilation up high by my imperfect fit of that plastic wrap so the ammonia can escape, plus if it stays dry there just isn’t any ammonia generated. Since it is heated frostbite isn’t an issue.
One of the problems you face brooding outside or even in your unheated garage is that the daily highs and lows can vary quite a bit. It’s really difficult to maintain a constant temperature. With a large brooder that’s not an issue at all. Just keep one end warm enough at the coldest temperatures, maybe even a bit too warm. Keep the far end cool enough in the warmest temperatures. Even straight out of the incubator mine are really good at self-regulating temperature themselves. Your goal is to give them a place to warm up when they need to but also give them a place to cool off if they need to.
There are all kinds of ways to provide that warm spot. I use heat lamps, others use heating pads, ceramic heaters or emitters, maybe a hover, just all kinds of ways. A hover is a shallow inverted box set up just high enough for the chicks to go under. It traps their body heat plus you normally provide a low level of heat in really cold weather so they can stay warm. Like the heating pad cave, people are often surprised to see them spend more time on top of it than under it. They really can handle the cold pretty well.
You mentioned it gets in the 20’s. Water can freeze at that temperature. I suspect that is outside temperature, not what you are seeing in your garage so it’s irrelevant. It’s what is in your garage that’s important. Yes, when you open the door to take your car in or out, assuming you do that, it’s not likely to get cold enough in there for water to freeze. Still, for those brooding where it is below freezing around the brooder, and that might be you, think about how you keep water thawed. I just use a shallow bowl filled with rocks to keep them from drowning when they walk in it (they will walk in it) and set that in the warm zone. Not directly under the heat but close enough so that it doesn’t freeze. With some other methods you may need to do something special to keep the water thawed. People do it all the time in their coops in winter s it’s just something to think about.
We actually do not park in our garage. I store my lawn mower and its a workshop mostly. It is conbected to the house but I dont mind walking the two feet to my car. I have small gravity waterers and feeders for my brooders. i even have the ones I use for my run the spring/summer with the nipples. Looks like drafts Re my biggest issue over the cold. The dust was so terrible last time and I really rather avoid it. If they are in the garage I can easily put some bedding in a tub and bring them inside for a bit for my kids to snuggle on them. The more we handle them the friendlier they seem to be. Even my rooster isn't aggresove towards us. (Now its hit or miss with my husbamd but he didnt spend time torturing him with lovins like the kids and I did.) Lots of great information. Thanks guys!