Well this is more of an economic question then a historical one but I will attempt to answer as best I can. After the Korean War South Korea was in worse shape then the North. The north had basically all of the heavy industrial capacity. So the government enacted a series of polices that mixed neo liberal economics with the more traditional Keynesian economics. To put it in simple terms they created state directed capitalism heavily based on exports. They had low wages and tax laws that encouraged foreign investment and at the same time they heavily invested in education. Over time with an intelligent population the government started to direct what are called chaebols or conglomerates in English. These were private companies that followed the orders of the government in exchange the government signed the loans to give them more capital. the biggest of these was Hyundai these companies mostly followed the same building up of complexity in producing. In the 60s it was textiles, 70s and 80s heavy industry and defence then in the 90s it moved to consumer electronics like Samsung. So in summary it came about because the government worked in partnership with previously existing companies found a gap in the market exploited it for cash and used that cash to build up more and more lucrative industries.