Why would it be hard to compile? Costs over supply chains are calculated all the time. And we don't have to look at IPhones, we just have to look at the subset of processes that emit non-neglible amounts of CO2 regardless of if they are the final product or not.
The problem is deciding categories, and what to account for what and duplicate accounting of same operation in different sources and differences in accounting and not everybody doing it the exact same way and lack of transparency for sub-components. Think about making steel for wind turbines, or transporting sand for base of wind turbines. Both actions should be accounted on wind turbines co2 emissions, but also they should be accounted for transportation/steel industry and construction. Everything is interdependent.
But lets put things in perspective what is ineffective, in Finland home electricity use is 7% of all energy products used in the country, and still people concentrate on that you should make marginal reductions in your electricity usage to reduce CO2 emissions.
