The 2 numbers provide only a rough indication of viscosity and only at 2 temperature extremes. The 2nd number is a viscosity indication at 100C which is more or less the operating temperature of an engine, or close enough. So the 2nd number simply tells you how thick an oil is when engine is fully warmed up.
The 1st number is an indication of viscosity at extremely low temperatures, around -30C (again more or less). So lower number like a 0W simply states that it is thinner at very low temps.
Therefore these 2 numbers don't tell you when happens in between, such as more common 0C or 0F, etc.
To make matters worse there is only W20 and W30 and W40 with nothing in between. If you used more significant digits then a thick oil like GC might be 0W38 and a thinner oil like PetroCan might be 5W28. That is why a nominal 5W30 PetroCan is called thinner than nominal 0W30 GC.
Now as temp drops from 100C (2nd number) their viscosity increases, but not at the same rate. So for most of the temp range say from 100C down to 0C the thinner PC will remain thinner. The say below 0C at some point PC viscocity will increase at a much faster rate than GC's and by the time you get to -30C reference temp GC actually end up being thinner.
The problem is that oil companies don't release much data beyond standard specs, so we have to guesstimate a lot.
What many are saying is that since most startups are not at such extreme cold temps but closer to 0F then 0W doesn't actually matter much. These days I am leaning more towards 5W30 and mixing some 5W20. Again, based on the limited data you have to go by CCS/MRV as the best indicators, far better than 0W or 5W numbers.
