umd lives in Santa Barbara. In winter rides, at his age, most of the time, I could ride with a regular fingerless glove there. For really cold mornings a thin liner glove would be fine. (If I was so lucky to live there.)
For other people, it depends on where you live. For sub-freezing weather, think about wind-blocking. Some gloves do this better than others. Gore-Tex membrane (which by contract requires sealed seams) does a very good job. Moose Mitts designed for RB handlebars do a really good job. Lobster mitts, windproof, and full mittens diminish dexterity, but keep your fingers together, for warmth sharing.
Always remember to address your head, torso and arms. If they are warm, your hand and finger arteries and arterioles will be more dilated, delivering heat to them for any given temp. On long rides, I carry a down jacket. If I have to fix a flat, and am not generating body heat, this is fantastic.
If no passive-insulative system is working use chemical hand warmers. Some gloves have zip-in pockets for these. These are especially nice if you have to deglove to repair a flat, and when you re-glove, your hands are useless. Give them 5 minutes, and you'll be ready to go. If you're hands are really frozen slip in a pair for your palms.
I don't want to shill here, but Outdoor Research Alti Mitts will keep your hands warm and warm them up fast after degloving, without active heating. although they can accommodate chemi-heaters too.. They are expensive. They compromise shifting dexterity. Not for everybody. But they are TOASTY in sub-zero weather with -10F or -20F or below windchilll. But at that level, Epic Designs pogies, on MTB only, are also a great option.
If you ride in a wide variety of temps, you have to get different glove kits, in the same way you have to go from ss jersey and shorts, to ls jersey or arm warmers and shorts, then go to bib knckers or knee warmers, then jacket and full tights, then layers, cap and neck covering, full-face balaclava with dual chest and leg coverings, shoe covers...