Man who built ISP instead of paying Comcast $50K expands to hundreds of homes

Depends on how old the apartment buildings are. If the apartment buildings themselves are already wired with fiber (or really good, recent coax) it might be a lot cheaper running a single bundle of wires to services the building. (Keep in mind that ideally you still have one fiber wire per apartment to sell the highest speeds to each apartment, so you aren't necessarily saving on number of cables for 100 apartments versus 100 detached single-family homes.)

Of course, the older the buildings are the more expensive it gets. Running a new line into a single family home is usually a single new hole from the local utility trench or utility pole, which often have existing rights of way and known contact points to do utility work. Running new lines in an apartment complex often requires opening walls and ceilings between, among, and inside units, which then consequentially means doing new drywall and repainting (and maybe high costs to color match historic paints). If the apartments are condos there's even more complex rights of way issues in needing to get the consent of individual unit owners for some of the work.

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.