It varies from municipality to municipality but I have lived in a couple places where I definitively know that the flow is:

1. ISP lobby county to do construction to improve infrastructure (e.g. Laying fiber as part of a road improvement)
2. Maybe the county calls a vote, maybe they just do it
3. County does all the work
4. ISP is the only one allowed to use said infrastructure because America

And from my own home improvement work, I would be shocked if the county did not have to be looped in for any significant work to the various drops that lead to housing blocks. I vividly remember being confused as to why there needed to be a county truck and a Comcast truck out by the box when they finally acknowledged my internet was fucked (mostly because I provided them three or four weeks worth of log data).

Like I said. I don't know how South Korea handles things. So I can't have a proper opinion on this. But I understand well enough that there are a significant number of steps between "I want fiber" and "comcast/verizon lets me give them money for fiber".

And if the issue is just significant load from twitch streaming (I would say Youtube Streaming, but I doubt most Koreans watch Ludwig and he is basically the only streamer that exists on that platform): I can very much see an argument for telling Amazon that they have to chip in for the infrastructure improvements. Because fuck Comcast (or, I guess, Korean Equivalent Of Comcast?) but also... fuck Amazon.