Late infall episodes, where material from the surrounding environment accretes onto Class II protoplanetary disks, are emerging as a potentially important but poorly quantified driver of disk evolution. Observed as filamentary streamers in molecular lines and scattered light, such late-stage accretion can perturb disk structures through localized shocks, density enhancements, and warps, yet its chemical consequences remain poorly constrained. We present NOEMA 1.2,mm line-survey observations of the AB Aurigae system, a structured young Class II Herbig disk that shows evidence of both ongoing infall and planet formation. We detected strong azimuthal chemical diversity: SO emission is enhanced in the northern disk near the inferred streamer--disk interaction region, while C_2H emission peaks on the opposite southern side. In contrast, CS forms a nearly axisymmetric ring, while HCN and HCO+peaknearthedustcontinuumoverdensityatthemainmillimeter−sizeddustring.Usingmulti−transitionrotational−diagramanalysesofSOandCS,wequantifiedtheazimuthalcontrastincolumndensityandexcitation.TheSO−brightsectorexhibitshigherrotationaltemperaturesandSOcolumndensities,whereasCSremainsnearlyaxisymmetricwithsubstantiallylowerrotationaltemperatures,suggestingthatthetwospeciesprobedifferentdisklayersand/orchemicalcomponents.ForC2H,potentialtemperaturevariationscontributetobutcannotfullyexplaintheobservedasymmetries.TheHCO+/H13CO+lineratiofurtherindicatesthatHCO+isopticallythickacrossthemolecularring,whiletheelevatedratioinsidethecavitysuggestsanenhancedgas−phase12C/13C,consistentwithisotope−selectivephotodissociation.Comparisonwithgas−grainchemicalmodelsfavorsgas−phaseC/Oratiosnearoraboveunity,withahighereffectiveC/OintheC peak near the dust continuum overdensity at the main millimeter-sized dust ring. Using multi-transition rotational-diagram analyses of SO and CS, we quantified the azimuthal contrast in column density and excitation. The SO-bright sector exhibits higher rotational temperatures and SO column densities, whereas CS remains nearly axisymmetric with substantially lower rotational temperatures, suggesting that the two species probe different disk layers and/or chemical components. For C_2H, potential temperature variations contribute to but cannot fully explain the observed asymmetries. The HCO^+/H^13CO^+ line ratio further indicates that HCO^+ is optically thick across the molecular ring, while the elevated ratio inside the cavity suggests an enhanced gas-phase ^12C/^13C, consistent with isotope-selective photodissociation. Comparison with gas-grain chemical models favors gas-phase C/O ratios near or above unity, with a higher effective C/O in the C_2H-bright sector. We discuss two plausible, nonexclusive origins for the observed chemical asymmetries: (i) infall-induced heating and desorption of O-bearing ices that enhance SO and lower the local gas-phase C/O near the streamer's impact site and (ii) planet-driven substructures and localized heating or enhanced UV irradiation that can promote hydrocarbon-rich chemistry on the opposite side. These results highlight that environmental accretion and planet formation can jointly imprint azimuthal variations in disk chemistry, with potential consequences for the compositions of forming planets.