How does Wi-Fi reception of wireless devices WORK?

I have not been able to find an operational explanation of how the new Nighthawk Rax120 improved TWO-WAY transmission between say the router and my iPhone. I do understand how this wonderful new router broadcasts/transmits further, say through two walls, so that now my iPhone can “hear”/ receive its signal. BUT, how does the improved router now “hear”/receive my iPhone’s signal/transmission through the two walls, since my iPhone has NOT changed in the least?