Hello @Fraser84
If your house has been built using construction materials that have trouble carrying 5GHz WiFi signals (and it sounds like the floors, at least, have been), then yes – wiring the access points via Ethernet is highly recommended. Here's a support article with the details: https://support.google.com/googlenest/answer/7215624?hl=en
But, the short version is that the primary unit (the one connected to your internet service) is the router & firewall, and it needs to be "upstream" from any wired secondary units, meaning they need to connect in to an Ethernet network that connects in through the LAN Ethernet port on the primary – they can't connect to a common "outer" network that might be created by your internet service equipment.
In addition, Google WiFi units all support this, and the Nest WiFi Router units also support it. But, the Nest WiFi Point units do not support it (they don't have Ethernet ports). So, you can have a Nest WiFi Router as the primary, with one or more Nest WiFi Router units added as wired secondaries, or you can add Google WiFi units as wired secondaries to either a Nest WiFi Router or Google WiFi primary. Technically, you can add a Nest WiFi Router as a secondary to a Google WiFi primary as well, but that doesn't necessarily make sense.
