Naturally where you live dictates what "winter" consists of. I live in Madison WI, and commute to work year-round, though not this year so far due to working from home. Here, it can get down to -20 F (-30 C) although not all that frequently.
I think if you're already outdoorsy, then you know what you need to wear for your climate when doing stuff outdoors for periods of maybe an hour or two. I think you can wear the same stuff as you wear for just generally being active outdoors, with some minor accommodations. For instance you can't wear a winter hat under your helmet, but you can wear a windproof cycling cap to cover your ears. You will need enough flexibility so you can turn your head and maneuver the bike. If a freak polar vortex comes through, and you're under-equipped for it, then sit it out and make a note of what you are missing for next year.
Lighting is important, since you may be riding in the dark. I commute both ways in the dark during the winter due to time zones, my work hours, etc.
My extreme gear is a downhill ski parka, insulated snow pants, heavy mittens (warmer than gloves), boots, neck tube, face mask, and when it gets well below zero, downhill ski helmet and goggles. This is also the stuff that I have worn outdoors at subzero temps for hours. My winter bike has studded snow tires and a three speed coaster hub, so I can operate the shifter in mittens. I have bar mitts as well, they are great. Another reason for a dedicated winter bike is that road salt is not friendly to bikes.
I don't wear all of that stuff unless I need it.
One feature of our climate here, is that as we come back out of winter, we can have length periods where it's quite cold in the morning, and warms up in the afternoon. And sometimes I don't guess right and end up getting too warm. So I have a basked attached to my bike, which accepts all of the garments that I remove on the way. Riders who are more athletic than I am can work up a sweat quickly in winter gear, and have to worry about soaking their clothing which then becomes dangerous. Being a slowpoke who doesn't sweat much, it's not a huge problem for me. Your body will dictate what adaptations you have to make.
Winter riding sounds like some kind of badass thing, but it's not much worse than walking. There are people who work full time jobs outdoors in places like Alaska, so you know that humans have figured this stuff out.
Make sure you own a decent floor pump for maintaining your tire pressure at home, and whatever you need to fix a flat while on a ride. There are lots of threads about choosing those things.
Last edited by Gresp15C; 11-27-20 at 11:55 PM.