I don't think you understood how CDS works either

The mortgages are pooled into MBSs, single mortgages blend into a giant pool (done by Fannie Mae), then this pool is divided up into trunks of sellable securities called either MBS or ABS. MBS/ABS are then sold to the Investment Bank and IB are then turning them around to Investers at large. CDS contract are written upon these MBSs, not on individual mortgages.