You can get boilers that generate steam for radiators and hot water for tap water.
If so - this might explain something that's been mildly irritating me. When looking for alternatives to fuel-burning boilers it seems easy to find positive stories about space heating - and much less info on water heating, to the point where it's starting to look like maybe some of these options might not be great. Water heating is my largest home energy usage and space heating very little because it is grossly overrated; if you are not really unwell an indoor temperature around 10 c is quite pleasant, so I tore my (rubbish) radiators out in 2017 and never looked back. This is making it surprisingly hard to figure out what replaces the boiler. If the marketing is aimed at pure heating boiler replacers, this difficulty is at least easier understood if not easier solved.
I would personally not like to live in 10°C but I prefer hot climates. 15°C I could do. How do you keep the temperature above 10°C? Or is the outside temp rarely below 10°C?
Outside wobbles around -2 to 8 in winter here, inside rarely less than 6, which is nippy but not dangerous. A not very insulated house with solid walls (this one's from about 1890 and has only had the easy things done) doesn't really sink to outside temperature if you live in it, cook, have appliances etc. There are plenty of heat inputs, just they do another job first. Hence my interest in water heat, I think it would start to be grim without it. Stove for a small number of really cold evenings, plus visitors. It would be a really bad idea to use it all the time but it would be hard to fit a radiator-type system to the job of "one hot room occasionally".
https://www.energy.gov/energysaver/heat-pump-water-heaters
But I think the general assumption is that if you live in a cold place, you'd heat water and your home at the same time so you're a bit of an outlier.
Many of them make reference to cooling the air at the same time as heating the water, which makes sense if you love in a warmer climate.
I still have a condensation problem in a few specific places I have to keep an eye on. It's not general.
I understand that american indoors heating is mostly done through HVAC. Since in that case the heater would be heating the air directly, you'd need a separate boiler. Heating water to heat the air makes sense when water is your transfer medium (e.g. hot water radiators), less so when the air is.
So I expect US homes usually have separate air furnace and boiler, whereas in e.g. europe where water central heating is common you'd usually have a combined furnace/boiler.