How did Keynes overthrow the theoretical assumptions of classical economics?

This is a step-by-step process, carried out in stages,

The first stage: After World War I, Britain's economy was out of balance for a long time and unemployment was serious, which Keynes believed was caused by economic contraction. Therefore, he strongly criticized the gold standard system before the restoration of Russia, arguing that it would lead to deflation and the imbalance of relative prices at home and abroad, thus making the prices of British exports too high, which was not conducive to competition, and this was the root cause of British economic difficulties. Therefore, he advocated that the government adopt a monetary management policy to regulate the economy through price control (inflation rather than deflation). At this time, Keynes has begun to deviate a little from the route of traditional economics.

The second stage: unemployment in the 1920s made him feel that the free economy was not fully self-regulated, and laissez-faire was not enough to bring about economic revival. In the face of long-term unemployment, he advocated increasing expenditure on public projects to increase employment opportunities and reduce unemployment.

The third stage: from 1929 to 1933, the worst, longest and widest economic crisis broke out in the history of capitalism, with economic depression and serious unemployment. Traditional economic theory can no longer explain various economic phenomena during the Great Depression, let alone provide "effective" countermeasures for getting rid of the crisis. It is in this state that Keynes devoted himself to the study of economic theory in order to seek measures to get rid of the crisis, and published the General Theory in 1936, which fundamentally shook the traditional economic theory and caused a revolution in economic theory.