What is full-duplex FDD after-antenna-architecture and how it works ?

hi guys, I wonder how TX and RX of full-duplex system work together.
As far as I know, the simple architecture likE

e .So if A transmit signal f1, receive f2 then B receive f1 and transmit f2. but A and B must have the same architecture and something make them transmit and receive the reverse frequency. in some documents, it is said that a switch is inserted before duplexer, like

. so, when A want to transmit, the witch is on, then A transmit f2, and receive f1. perfectly match with B, transmit f1 and receive f2 (because the switch is off). But B also transmit signal like A, at same time, so why the switch of B is off. it should be also on.
can you help me solve this confusion. it make me crazy.! tks in advanced.