Routes used in default routers provided by ISP's

I would like to know if the default routers that the ISP's provide come hardcoded with default path or is it something that come via "updates".

What I really would like was to buy a better router to my home but the question pops up: How does the new router will know what is the default gateway?

I don't have much knowledge about WAN's. I really would like some answer giving an insight how the default router from the ISP knows which path it has to take.