Layering clothing for Scottish winter climbing is really difficult. Very changeable weather and stop/start activities like climbing and belaying mean that clothing has a really hard job to keep you warm and comfortable. Also, the worst weather is often found a long way from the safety and respite of the car or civilisation: this isn’t the Alps where a hut or lift might be just round the corner!
There are plenty of different approaches on what to wear for Scottish winter climbing layers. Some people forego the traditional layering systems altogether when climbing in Scotland but most people have a pretty simple system that combines a baselayer, midlayer(s), a shell jacket, and a belay jacket; whilst on their legs they’ll wear baselayer tights or slightly thicker legwear, and then shell pants. The damp weather of Scotland is worth particular consideration, and so down jackets are generally not recommended as belay jackets, and you are usually better off with a synthetic jacket.