I really am not sure what your question is.
I live in northern part of USA, most of winter is a really bad time to cycle outdoors. Yesterday we had freezing rain most of the day, the wind is howling today and it is about 5 to 10 degrees C below. I do have studded tires on my Nomad for winter riding, but there are a lot of days that outdoor riding in winter would just be out of the question.
So I usually spend 3 sessions a week, one hour at a time at a health club on an exercise bike. Plan to do that later today. I e-mailed to myself a bunch of news articles that I can read later on a 7 inch android tablet while on the exercise bike. I use a heart rate monitor, usually try to get my heart rate up to about where it is when I cycle for exercise.
I also have a bike on an indoor trainer. That I sometimes use at home for exercise.
https://kurtkinetic.com/products/kinetic-magnetic/The indoor bike, it is an old mixte frame bike that a neighbor put in the garbage, I recovered it, re-greased the rear hub and bottom bracket (which also needed new ball bearings), and did a few other tweaks, that is my indoor trainer bike at home. I put a different saddle on it, different pedals, might have put a new chain on it. Nice setup and the price was right.
There are indoor spin classes at some health clubs, I have never participated in one of those. I think there also are internet spin classes too but I have no interest in that.
I am not a racer or competitive cyclist, I just do this for exercise.