When you perform (intensive) activities outdoors in different climatic conditions, the ancient art of adjusting the layering of your military clothing becomes your smart thermostat.
This allows you to control comfort by putting on and taking off layers as your activity level or weather changes.
How to tackle: To understand layering your clothing for outdoor activities, you need to know the function of each layer:

Base layer: wicks sweat away from your skin
Middle layer (insulating and uniform layer): retains body heat to protect you from the cold and further wicks away sweat to the outside
Outer layer (shell layer): protects you from wind and rain and further wicks away sweat to the outside.
Even if you don't wear all three layers at first, it's a good idea to consider which layers to bring or not, depending on the weather forecast and the activities you'll be doing : you can take off layers as you warm up, but you can't put on layers you didn't bring. So also provide enough space in your daypack and/or backpack to carry and/or store these clothes.
At BDCS, we work with different layers that complement each other. Some layers everyone gets, some layers you only get in case of operations. If the latter, 'who gets it?' is defined by ACOS O&T/J4 :

Layer 1: close to skin underwear 100% merinowool
Layer 2: winter underwear (zip shirt and long john) 76% merinowool
Layer 3: Wool Terry Jacket 400 with 70% merinowoo (can also be layer 4 depending on user's choice)
Layer 4: Field or Combat shirt (can also be layer 3, depending on user's choice)
Layer 5: High Loft Jacket (& pants for COMBAT)
Layer 6: Tactical Jacket
Layer 7: OLIVE GREEN foul weather clothing. Both from GORE-SIAMIDIS in case of WET/COLD or as an extra wind-stopper.
Layer 7: Foul weather clothing GORE-TEX PYRAD® FR by Siamidis | GORE-TEX

How should I wear the different layers of clothing from the military chain combined?