Does VCO have to be used in a PLL to implement LO?

I am new to RF transceivers and I' in the process of designing the RF sub-circuits. My question is, is the VCO always used in a PLL to implement a LO? For example, implementation of a PLL for high frequency applications (> 20 GHz) might be very challenging. Do people use VCO directly as a LO in RF systems?

You can use a low frequency PLO and then use a frequency multiplier to get your high frequency.

In any PLL system you need a RF VCO as this oscillator is locked to a lower reference frequency harmonic.
A typical microwave locAl oscillator PLO is a VCO at say 5 GHz and a 100 MHz crystal oscillator is used as a reference. A comb multiplier generates many harmonics of the 100 MHz, only the 50th is used in the phase detector to generate the tuning voltage for the 5 GHz VCO.

VCO is used definitely how else you can ensures that he error is accounted for, as said a multiplier can be then used to raise the frequency