As others have said, your cable modem may have a router already built into it. Usually, these also include WiFi. So, if your cable modem has WiFi, it already has a router built-in. If it doesn’t, you need a router. Most people use a WiFi router or a mesh setup with multiple access points is common these days.

However, you definitely do NOT need a smart managed switch - especially if you don’t understand what a router does. If you need a switch, just buy a cheap unmanaged giga switch.