National health insurance application as working holiday

Hi everyone ! I've started my working holiday a few days ago !
Today, i went to Nagasaki city hall, the first city i'm staying in, for one month.

I applied to the national health insurance, an old lady took care of my case, and her English level was kinda at the same level than my Japanese, not to blame her ! We tried the google translate way but she was not articulating enough so it wasn't really helpfull.

We managed to do it, but i've a few questions.
I was expecting to have to pay 5000 yen or aroundish per month, but i've for a paper talking about national pension system and 17000 Yen per month.

Is that mandatory to subscribe to this in order to use the health insurance ?

She said too that i wont have to pay the bills i will receive. Didn't really understand this part. Should i just ignore bills, or go to the city hall to say i dont need to pay them ?

I'm a bit lost and i feel sad for that lady that did her best to help me. Atleast i gave her a gift from France, i was expecting to make someone day a bit hard today, so i prepared a gift aha.

Thanks in advance, can send some documents for more informations if needed !