Napoleon had hundreds of thousands of conscript soldiers without France being industrialised so how did it cause mass conscription?
In France it was the production of the revolution. That swept the aristocracy and church out of power in addition to the monarchy, and they were the source of military leadership (by law: a man had to show 4 quarters of nobility to become an officer, which means all four grandfathers had to be aristocrats - a closed system indeed) and legitimacy. To compensate for the sudden lack of any army in the face of invasion by every major monarchy nearby (Prussia and Austria being the two major antagonists), the revolutionary government proclaimed the first great
levee en masse to form a new military. They had spotty leadership and no training to speak of in the traditional 18th century sense, but there were hundreds of thousands of them. Napoleon kept the conscription system and added professional leadership and training.
Other states, interestingly, did not adopt conscription right away: Britain did not regularly conscript soldiers until 1916. Only Prussia, which after being conquered by the French in 1806 by treaty was not allowed to form a large army, adopted a form of conscription to sneak around the treaty: they drafted the allowed number of men each year, trained them and then sent them home and replaced them so that they fairly quickly built up a trained reserve that allowed them in 1813 to put a quarter million men in the field, more or less ('more or less' because they were still seriously short of weapons, uniforms and all other equipment for their forces and remained so until after the wars were over at Waterloo).
That actually became the pattern for the European states (except for Britain) by the beginning of the twentieth century: draft X number of men (generally about 1/3 of the total force) every year, keep them for about 2 years to train, then send them into the reserves and draft more. Consequently, the government in peacetime only paid to have 3X troops in uniform, but could almost instantly raise 10 - 20X troops from reservists as soon as war was declared.
Result: in late 1914 6 million armed men marching across northern France and quickly running out of room to march without marching into machineguns, rifles and light artillery firing in their direction and bringing all movement to a bloody halt.