I read on the Incometax site that:
A resident senior citizen who does not have any income from business or profession is not required to pay advance tax and he can pay the entire tax by way of self-assessment tax. For computing the limit of Rs. 1 lakh (as specified above), the self-assessment tax paid by a senior citizen on or before 31-07-2021 shall be deemed to be the advance tax.
So I presume if the total tax of senior citizen is around say, Rs 150000/-, and if one pays Rs. 100000 on or before July 31, 2021, then this payment in respect of the senior citizen will be treated as advance tax and if the balance tax (in this case Rs 50000) is less than Rs. 100000/- then no interest will be charged for delayed payment. But I would need this to be clarified.