I have an insulated roof, walls, and doors. It helps me to keep the garage cooler in the summer and warmer in the winter. It’s also attached to the house and gets some warmth from there. Sometimes, when it becomes too cold, I use an electric heater to warm up the air a bit.
But most of the time I just dress warmly when I go there. I turn on the heating only when I have to repair the car for a long time. Thanks to properly installed wooden insulated doors, almost no warmth leaves the garage, and it’s always fine in winter. It’s never bitter cold, and I am mostly fine with that.