In order of increasing chill I start with fleece lined Craghopper trousers, then a set of Paramo trousers, then salopettes. I can add a thermal base layer as well. The coldest I know I have been out in is - 28C plus wind chill. Never hit -40, which I believe is where the thermometers cross, can’t say as I fancy it though.
I do remember looking out the pub window in Brough in the depths of winter playing “spot the Geordie” as folk passed through on their way to Blackpool from Tyneside.