HELP!!!VCO design

Dear All,
I have attached the three diagram of VCO .No.1 is VCO for NMOS only tailed bias VCO. No2 is differetial VCO . No3. is Oscillator from cross couple inverter.

Question: why they use 4 terminal NMOS as varactor ?what is the advantage of this .?

Question :In No.3 , I know the top PMOS is cross coupled and what is the function of NMOS below. Why he choose that layout and advantage?

I will be waiting your answer .If you can help me, very thankful.

Thanks..

VCO-3 is same as VCO-4