Hi @Nianyu Jiang,
1. I believe this is high pass filter to move the offset. -- That is correct.
2. I believe this is Op-Amp, but why do we need this? -- This op-amp is arranged as a voltage follower, to help further stabilize the signal.
3. I think this is a switch or multiplexer, but why do we need this here? & 4. Same as part 2. -- This is part of the feedback amplifier that goes from the received signal (after passing through the device under test) to internal ADC, detailed further on page 15 of the AD5933 datasheet and shown again in Figure 24 on page 18. As this user mentions in their post, driving SEL to ground sets the feedback resistor (what the mux/switch is selecting) to 100 kOhms and setting SEL to Vdd sets the feedback resistor to 20 Ohms. The choice in the feedback resistors (as well as the gain factor and output excitation voltage range selected in the Control Register) affects the result which needs to stay within the internal ADC input range of 0V to Vdd. To figure out if the gain through the system is within the ADC voltage input range, follow the calculation presented on page 18:
You'll note that this inherently limits what can be reliably measured with regards to the unknown impedance depending on what settings you have chosen, which is also noted in the user post I linked to earlier.
Let me know if you have any questions.