Hello @gluefingers
Google WiFi is a router (and firewall). It also supports additional Google WiFi units that expand the coverage area beyond what the primary unit can support by itself. So, if you have an existing router that provides WiFi, a Google WiFi system would replace that device unless that device is also your cable modem! Basically, the primary Google WiFi unit (the first one you set up) will be a router+firewall, and it needs an Ethernet connection to the internet. So, if you have an existing combination modem+router+firewall+etc device, you could keep using that, but put it in "bridge" mode so it's acting like a modem only, and connect a Google WiFi unit to that. Or, you could replace it with just a modem and connect a Google WiFi unit to that instead. Either way, the new Google WiFi unit will provide WiFi to clients and has another Ethernet port you can use to connect wired devices. If you configure the new Google WiFi system to have the same network name and shared password as your old system, your existing client devices should all connect to it transparently – no need to reconfigure them. You definitely don't need to replace them. I have a bunch of Amazon Echo devices of various kinds as well as devices from Apple, Google, and smart plugs from TP Link all connected to my Google WiFi system.
