Multiband light curves of AT 2021biy. The dashed vertical line indicates the r-band maximum light as the reference epoch. The epochs of our optical spectra are marked with vertical blue solid lines, while the two epochs of NIR spectra are indicated with vertical red solid lines. Upper limits are marked by empty symbols with downward-pointing arrows. For clarity, individual-band light curves are shifted with arbitrary constants reported in the legend. In most cases, the data-point uncertainties are smaller than the plotted symbols.