All of them. It isn't just that it takes twenty years to train a software engineer, 30 years for a doctor or a lawyer.
It is also the tools we need (I don't even dare think about the complex physics that has to be mastered to make the microprocessors that enable me to encode bits that can be sent halfway around the world so you can see this comment), the tool required to built those tools, the food we eat (easy to make perhaps, but it needs to be brought and sold, transported, stored and finally sold to us consumers), etc.
The interconnections in this world are truly mind wrecking.
It is also the tools we need (I don't even dare think about the complex physics that has to be mastered to make the microprocessors that enable me to encode bits that can be sent halfway around the world so you can see this comment), the tool required to built those tools, the food we eat (easy to make perhaps, but it needs to be brought and sold, transported, stored and finally sold to us consumers), etc.
The interconnections in this world are truly mind wrecking.