Electronic structure complexity and extremely large magnetoresistance in antiferromagnetic semimetal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>SmAgSb</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math>

${\mathrm{SmAgSb}}_{2}$, a layered magnetic semimetal in the tetragonal $RT{\mathrm{Sb}}_{2}$ family ($R=\mathrm{Y}$, Sc, rare earth; $T$ = transition metal), is known to exhibit extremely large magnetoresistance (XMR) below its antiferromagnetic (AFM) transition temperature. In this work, we present a comprehensive investigation combining magnetotransport measurements, density functional theory calculations accounting for electron correlation, and angle-resolved photoemission spectroscopy. Our results reveal a complex electronic structure characterized by a multiband Fermi surface and intricate magnetic ground states. We demonstrate that simple two-band models, previously employed in the literature, fail to consistently describe the observed transport phenomena. Notably, we report an XMR of approximately $25\phantom{\rule{0.16em}{0ex}}200%$ at 2 K under a 14 T magnetic field, significantly exceeding earlier reports for this material family and rivaling the performance of prominent nonmagnetic XMR systems. This pronounced enhancement below ${T}_{N}$ suggests that the XMR originates from a combination of multiband electron-hole compensation and enhanced magnetic scattering in this correlated AFM semimetal.