In Finland and most of Europe have 230V one phase, 400v in three phase. And bare single phase subscription haven't even been available new houses for at least 45 years any more.

But if you buy an old summer cottage further away from permanent living areas it may well be that a) you don't even have grid there or b) if you have it's single phase and three phase upgrade would be too expensive because you are being billed building cost for that work all in front.

Using that single phase for sauna stove needs then so much that it's not allowed by code or if you would be able to convince some electrician do some kind fo switching other devices off when stove is on most perhaps do not like to pursue that and choose wood stove their sauna instead. That's known working solution and remote location it's also a secondary heat source incase grid were down due some storm fallen trees on wires which mess cleaning takes several days etc.