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\object{SN 2021aefx} is a normal Type Ia Supernova (SN) with red excessemission over the first $\sim$ 2 days. We present detailed analysis of this SNusing our high-cadence KMTNet multi-band photometry, spectroscopy, and publiclyavailable data. We provide the first measurements of its epochs of explosion(MJD 59529.32 $\pm$ 0.16) as well as ``first light'' (MJD 59529.85 $\pm$ 0.55)associated with the main ejecta ${\rm{^{56}Ni}}$ distribution. This places ourfirst detection of SN 2021aefx at $\sim -$0.5 hours since ``first light'',indicating the presence of additional power sources. Our peak-spectrum confirmsits Type Ia sub-classification as intermediate between Core-Normal andBroad-Line, and we estimate the ejecta mass to be $\sim$ 1.34 $M_{\odot}$. Thepre-peak spectral evolution identifies fast-expanding material reaching $>$40,000 km s$^{-1}$ (the fastest ever observed in Type Ia SNe) and at least twodistinct homologously-expanding ejecta components: (1) a normal-velocity(12,400 km s$^{-1}$) component consistent with the typical photosphericevolution of Chandrasekhar-mass ejecta; and (2) a high-velocity (23,500 kms$^{-1}$) component visible during the first $\sim$ 3.6 days post-explosion,which locates the component within the outer $<$ 16\% of the ejecta mass.Asymmetric, subsonic explosion processes producing a non-spherical photosphereprovide an explanation for the simultaneous presence of the two components, aswell as the red excess emission via a slight ${\rm{^{56}Ni}}$ enrichment in theouter $\sim$ 0.5\% of the ejecta mass. Our spectrum from 300 days post-peakadvances the constraint against non-degenerate companions and further supportsa near-Chandrasekhar-mass explosion origin. Off-center igniteddelayed-detonations of Chandrasekhar-mass white dwarfs may be responsible forthe observed features of SN 2021aefx in some normal Type Ia SNe.

Origin of high-velocity ejecta and early red excess emission in the infant Type Ia supernova 2021aefx