For my core, I wear a long sleeve breathable wicking layer (a light long sleeve bike jersey is fine). If needed, a thin fleece jacket, and a windbreaker (hard shell?) outer layer. I really only need the fleece in the coldest conditions (below -7 C). Layers are good. Add and remove as needed. A little cold at the car will be perfect on the trail. Comfortable at the car will mean too hot on trail, and layers will have to come off. I usually bring an extra layer in my pack, in case of having to stop for a breakdown, and having to walk out.
I use a helmet liner inside my helmet, but once warmed up, rarely need it (Probably below -5 C). Below -10, a balaclava.
I usually ride with shin pads, and can get to below zero with shorts and shin pads. Near zero, I wear a regular set of tights. Below (-10 C), I may add a set of nylon pants over the tights.
My hands often get cold for the first 20 minutes of a ride. After that, I've usually warmed up enough that my hands are comfortable for the rest of the ride. I have a set of winter cycling gloves that I've used hand warmers in to get me through the first part of the ride until I warm up. Sometimes, I'll then switch to normal cycling gloves after I've warmed up. Now, I have heated grips, which work really well to keep me comfortable, and I can usually wear normal cycling gloves. My wife has battery-heated gloves that work well for her.
My feet (especially toes) have been often cold for a whole ride. Normal cycling shoes are meant to be ventilated for cooling, so don't work well in winter, and if you ride clipless, the cleat in the bottom acts like a heat sink, sucking the heat out of your feet. I usually switch to flat pedals and a hiker shoe when the snow hits. To keep my feet warm, I've had the best luck with neoprene booties over my shoes. They've worked well, but are a real pain to get on and off. Last winter, I tried just toe caps to break the wind over my toes. Much easier to get on and off, and easy to stow in a pack. They seemed to work well. I also switched to an oversize shoe to give more room for socks and to let the blood circulate. This year, I have battery-heated insoles that I will try out.