That setup won't work in the way you'd probably like it to. The mesh protocol only runs over WiFi. So, when you connect a secondary via Ethernet, it has to ignore the mesh in favor of the wired connection. So those more distant mesh secondaries will not be able to take advantage of the wired secondaries being there in any meaningful way. In addition, the mesh protocol itself prefers fewer hops to each destination. So those distant mesh secondaries are likely to talk directly to the primary if they possibly can – even if that means they slow way down to do so. So, wire the secondaries you can, but place the primary as close to the center of your home as possible with the mesh secondaries close enough to get a strong 5GHz connection to it. You may not want to deploy all of those mesh secondaries, either – 5 is the recommended maximum.
So if I install a point-to-point bridge from a puck to another puck (effectively a backhaul connection) to cover the interior of a building that is behaving like a faraday cage, that puck will function as a localized wifi access point? And if I connect it to the primary puck it should perform well in that role?
Yes, a P2P bridge connection from the primary to a distant secondary should work fine, as long as the P2P connection is solid. However, that (effectively "wired") secondary will not be part of the wireless mesh connection at that point, so any other wireless secondaries will still need to be close enough to the primary to get a strong connection for themselves.
I have a related question: two Nest WiFi routers are wired together in my network, the primary at the center of the house. The second router is wired to it so it acts as a "mesh" point since this old house has a lot of construction interference. It sounds to me if I tried add any more WiFi points (in this case Google WiFi points), those would only mesh to the primary and not the secondary even though they are wired together. I was hoping additional points would mesh to the secondary to "extend" the mesh network (in this case to the backyard) but it sounds like that is not how they act unfortunately.
Correct – a wired secondary doesn't really extend the WiFi mesh in any useful way. But, if you have an older home with solid interior walls, then adding more wired secondaries would be good advice. An inexpensive (unmanaged) Ethernet switch on the primary Nest WiFi Router's LAN port makes more ports available to connect all sorts of things, including Google WiFi units added as secondaries. Just set them up as mesh secondaries somewhere close enough first, then, once they're working properly there, move them to where you want them to be and connect their WAN Ethernet ports to Ethernet cables running into your wired network rooted in that Ethernet switch. They will all stop participating in the WiFi mesh, but will still provide connectivity for nearby devices and carry that traffic back to the primary via Ethernet.
@MichaelP thanks for the insight, makes sense. I'm not sure what I'll do in this case; there's not really any good way to get wires to the other mesh points, I think I'll have to dig in and make do with the two routers just wired together.
Clients operate the same way regardless of how the points are connected to each other or to the primary. They decide which access point to connect to and when to switch between them (and which band – 2.4GHz or 5GHz – to connect to, and when to switch between them). Some clients do a better job of this than others. But, that's a whole different (and equally complex) topic.
If you add a single wireless point to a system that has all wired access points, it will technically talk to the wired points, but not in a way that's particularly useful under most conditions. So, yes, I usually recommend thinking about a mixed wired/mesh system like this as building a pure mesh system (following the recommended placement rules) as well as building a wired system (following the required connection rules). In short, make sure the one wireless mesh point is close enough to the primary (one or two rooms away, typically) to get a strong connection directly to it.