I have Internet to the house with newly installed at&t fiber. I now have an Eero 6+ mesh network connected, as well at the at&t Wi-Fi network (nothing is hard wired). Speed test shows 1.3gbps to the house, about 360 Mbps through at&t Wi-Fi, and about 150 on the Eero WiFi.

My question is if the Wi-Fi networks are essentially individually maxed out, with lots of left over bandwidth? Would it optimize the network to connect things like smart TVs and Xbox to one (at&t).. And then use the other for other connected devices like cell phones and computers? Or do they all end up " borrowing" from each other anyway?

If that does make sense, how does a guest Wi-Fi come into play?

I would setup as pass through and use eero only, hardwire them back to main eero if you can.

Second speeds depends on which devices you are testing from. IE WiFi 6 capable devices and how close to the eero you are.

I can get 750/750 from a bedroom away from my iPhone 15 ProMax.