GSTR-3B is the summary return where you actually pay tax. Outward liability flows from your filed GSTR-1; eligible input tax credit (ITC) is matched against GSTR-2B. Net cash liability transfers to the electronic cash ledger.
Due 20th of the month following the tax period. Late fee: ₹50/day capped at ₹5,000 plus 18% interest on tax not paid by due date.