The layering system refers to technical properties, not to individual, single items of clothing.
base layer: something wicking moisture away from you
insulation: something keeping warm air around you
water/windproof: something keeping water and wind out
Those can be omitted when not necessary, combined, when relevant, or multiplied if needed.
For example, ...
For extreme cold, I will put more emphasis on the extremities. For example, I might add:
Thin gloves as inner liners to gloves or mitts.
Skull cap or headband under regular hat.
Thin inner sock under heavier outer sock.
Bandana, gaiter, or similar to cover neck and face.
Goggles for windy conditions.
Keeping hands warm is always my biggest challenge, and ...