What are the reasons for the most important changes in economic development at the beginning of the 20th century?

One of the most important reasons for the development of the economy was the absence of major military operations in the European part of the continent, as a result of which the number of the world’s population grew.
Due to the active development of scientific and technological progress, the emergence of new industries began.
Due to the fact that education has become more accessible, the number of skilled workers has increased.
Most countries have moved from an agrarian-industrial system to an industrial-agrarian one.
The population began to move to cities.