GSTR-3B is a monthly (or quarterly under QRMP) summary return. Unlike the invoice-level GSTR-1, it reports consolidated figures: total outward supplies, input tax credit claimed, and the net GST payable, which is paid at the time of filing.
The output liability in GSTR-3B should reconcile with GSTR-1, and the ITC claimed should reconcile with GSTR-2B — mismatches are a common trigger for notices.