Comment by VTimofeenko

To be honest, I only have second-hand experience with running Internet lines in a bunch of Soviet-era apartment blocks. On a lot of building designs there's typically a drop going through all floors that exposes the electricity meters in a common area of the stairwell. The cable would go in either through the underground utility way - most likely electricity or heat lines (central heating FTW) or by air from a neighboring house. There would be a switch in the attic where the connections from apartments would terminate.