Hi and welcome,
There are many different brands of "indirect hot water heaters", The brand I have is Amtrol.
Here is a link that has a nice cutout of how this system works.
This link is broken, either the page no longer exists or there is some other issue like a typo.
It is basically another zone on your hot water/steam heating system. The hot water from the boiler (or HW coil in your stove) circulates through the indirect heater where there is a heat exchanger that transfers heat from the heatings system's hot water to your domestic hot water.
I was told they cost ~$1,000, but mine came with the house.
Does this answer your questions?