Answer: You Actually Need Both, But Both Ethernet and Modems are Often Bundled Together
Back in the day, the ISP would come and install a cable modem to deliver you internet. You would then have to purchase a router to connect to that modem. Fortunately, for many residential users, the cable company provides a bundled device that brings in the internet and then outputs an ethernet jack so that you can connect your devices directly to the modem/router. They even provide you WiFi. There are some downsides though, the features on the router they give you are pretty limited, and power users often purchase their owner routers and then ask the cable company to set up the modem in “bridge mode.”
Regardless, if you decide to purchase internet in the vast majority of cases the ISP provides all of the necessary equipment.
Cellar Door solutions can help with your cabling installation needs