If we’re talking about, I guess you call them condominiums or blocks of flats, they are most likely heated by a local so called ТЭЦ (heat-electric generating station, which uses coal or natural gas as a power source) or just a heating plant which generates heat only, distributed then through a net of hot water pipes running underground. By the way the pipes, made of iron, made in Russia and thus made not to last quite often burst out, this time generating nothing more than problems, like hot water puddles, heat delivery disruptions and worst of all ruined roads and sidewalks left after blow-outs are mended.
There are houses of course, family owned houses, where heating system works differently. It utilizes a gas heated boilers and in rare cases electric heated boilers, this time the cost of heating rises threefold or even more, so electricity is only used when there’s no alternative or in other words when there no gas distribution system available.
Others may have already chipped in by the time I‘ve delivered my post, so let it be an add-on to what they might have missed. ))