Looking here:
https://www.bakom.admin.ch/bakom/en/homepage/telecommunicati...It seems Swisscom owns some lines, but not all of it. Like some of Swisscom Internet goes on lines that is not owned by them.
I guess a lot of the households in the mountains and other large terrains do not have fiber.
If you look here: https://map.geo.admin.ch/?topic=nga&mobile=false&lang=en&bgL...
Plenty of households have >=100 Mbit/s Internet. This map pretty much covers the same area as "where most Swiss people live."