You don't even necessarily need to run Ethernet. For example, my parents house was built without Ethernet cable runs to every floor, but they ran coax cables to

You don't even necessarily need to run Ethernet. For example, my parents house was built without Ethernet cable runs to every floor, but they ran coax cables to basically every room for TV. Thanks to MoCA (Media over Coax Alliance) you can route your Ethernet over Coax, and thus we have a really stable and performant (> 1GbE) Ethernet connection to every room. Wireless repeaters/mesh doesn't really work well when you have reinforced steel-concrete floors and walls.