This circuit is a 2-to-1 multiplexer. It selects one of two inputs (based on the select input at the bottom right) and outputs it at the bottom. It uses two tri-state buffers; the implementation of each buffer is shown in full. When the select input is low, input 1 is used. The buffer on the left is switched on, and the buffer on the right is put in a high-impedance state where it does not have any effect on the output. When select input is high, input 2 is used.
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