Questions regarding insurance and pension for foreigners

Hi im an international students studying in Japan currently, i have questions regarding tax. Last year i got a bill that told me to pay for their national health insurance tax, even though i dont have a job here i have 0 income. I paid around ¥17000-19000 for the whole year, yesterday I got a letter that told me to pay ¥30000 for 6 periods (im expecting this means months). I also got a bill that says by my understanding a chance to refund my tax that i paid last year for the insurance which was ¥17000-19000. My question is 1. As a foreigner who has no income, am I obligated to pay these tax? I dont think it makes sense for a country to file tax to someone that has no income (sorry if im wrong, please correct me, i might have gotten my knowledge about tax wrong) 2. Why am i told to pay a different ammount than the one i have last year? Thankyouu, sorry if i might have offended or triggered someone