My cold weather spd shoes are sized to handle a medium weight wool sock, have a felt pack in the bottom, and have had the cleat sealed against water... I can put shoe covers over those and be fine to -10C / 15F but it is far easier to throw on my Asolo hikers which are waterproof, windproof, and toasty down to -18C / 0F.

After that it is my pack boots which are good to -45C

Keeping your legs and ankles warm and allowing wiggle room in your boots is key to keeping your feet warm and there are some folks (like my wife) who just suffer from cold hands and feet despite following all this advice. I have a friend who wears boots with a -80C rating for weather I'd be wearing shoes since he has issues with keeping his feet warm.