India's current population of ~1.4Bn

Hello to all Redditors of this group.....trust all are doing well. The last official population census (comprehensive and encompassing each and everyone, I suppose) happened in India during 2011 and reported population was \~1.21 Bn. Currently in the public domain/media I come across population in India to be 1.4 Bn. How is this 1.4 Bn arrived? what is the logic behind this number? Considering the porus borders we have on all sides of India, aiding unchecked illegal migration , I am doubtful if whether 1.4 Bn is meaningful and the magnitude of understating it. Any insights on the above would be helpful... Tia