Below 20 GWEI the blocks are almost all full, may I know the reason here.
Where did you get this info from?
.as per my understanding if we set lower than 20 GWEI that WILL have to wait in line ??
Probably, but fee estimation is not an exact science.
It is possible that of you use a lower gas price your transaction will still be processed fairly quickly (even though not as fast as when you use a higher gas fee)