Hello @Fouclo
The Google WiFi units run an implementation of the IEEE 802.11s mesh protocol over a (hidden) WiFi network that shares the same 5GHz radio that is used to serve 5GHz WiFi clients. In any mesh network, the key question is how a node decides which node to send a packet to next. They need some "metric" to make that decision, and in this case, they use "fewest hops" as the metric. So, for your outdoor unit, if it can get a connection directly to the primary, it will use that rather than going through the intermediate unit in the bedroom – even if that means shifting the transmission down to a slower speed. If it were farther away – far enough it couldn't get a direct connection to the primary at all, it would go through another unit that it could get to. However, this would likely be pretty slow, since the data would have to be transmitted over that same 5GHz channel twice, and given the hypothetical scenario, neither of those transmissions would be happening at a high speed. So, will it "daisy chain"? Yes, under some circumstances. But, you really don't want it to.
This is why the optimal placement advice for a mesh-only system is to put the primary in the center of the coverage area (and, if possible, up high and away from obstructions) with secondary units no more than one or two rooms away from the primary. They should all be close enough to get a strong 5GHz connection to the primary. From there, they will provide 2.4GHz and 5GHz coverage to more distant clients.
Having too many secondaries can cause performance issues (since they have to share the 5GHz radio between client and mesh traffic, they all have to be on the same 5GHz channel – not that there are actually all that many to choose from anyway).
Connecting secondaries back to the primary through a wired network can let you put a secondary just about anywhere, but that is a different discussion (and building mixed wired / mesh networks is even more complex, since wired secondaries don't extend the mesh). But, for getting coverage in outbuildings (like the one in your wooden box outside), wiring a secondary is by far the most reliable method. If you can't run a physical wire, there are wireless bridges that have directional antennas that can be used to create a virtual Ethernet connection. I know people have used these successfully in the past, but they are significantly more complex to set up.
I hope some of this "under the covers" detail helps you understand what you're seeing in your mesh test results and may help you experiment with some alternative placement options.
