The　huge　amount　of　data　produced　in　the　6G　networks　not　only　brings　new　challenges　to　the　reliability　and　efficiency　of　mobile　devices　but　also　drives　rapid　development　of　new　storage　techniques.　With　the　benefits　of　fast　access　speed　and　high　reliability,　NAND　flash　memory　has　become　a　promising　storage　solution　for　the　6G　networks.　In　this　paper,　we　investigate　a　protograph-coded　bit-interleaved　coded　modulation　with　iterative　detection　and　decoding　(BICM-ID)　utilizing　irregular　mapping　(IM)　in　the　NAND　flash-memory　systems.　First,　we　propose　an　enhanced　protograph-based　extrinsic　information　transfer　(EPEXIT)　algorithm　to　facilitate　the　analysis　of　protograph　codes　in　the　IM-BICM-ID　systems.　With　the　use　of　EPEXIT　algorithm,　a　simple　design　method　is　conceived　for　the　construction　of　a　family　of　high-rate　protograph　codes,　called　irregular-mapped　accumulate-repeat-accumulate　(IMARA)　codes,　which　possess　excellent　decoding　thresholds　and　linear-minimum-distance-growth　property.　Furthermore,　motivated　by　the　voltage-region　iterative　gain　characteristics　of　IM-BICM-ID　systems,　a　novel　read-voltage　optimization　scheme　is　developed　to　acquire　accurate　read-voltage　levels,　thus　minimizing　the　decoding　thresholds　(in　dB)　of　protograph　codes.　Analyses　and　simulations　indicate　that　the　proposed　IMARA-aided　IM-BICM-ID　scheme　and　read-voltage　optimization　scheme　remarkably　improve　the　convergence　and　decoding　performance　of　flash-memory　systems.　Thus,　the　proposed　protograph-coded　IM-BICM-ID　can　be　viewed　as　a　reliable　and　efficient　storage　solution　for　the　new-generation　mobile　networks,　such　as　Internet　of　Vehicles.