Coordinates of the initial candidates (Section 4) selected from LSST DP1, VHS, and CatWISE (gray circles) within the Extended Chandra Deep Field South (ECDFS; left), Euclid Deep Field South (EDF-S; middle), and a field at low Galactic latitude (Rubin SV 95 -25; right). Orange stars mark the ultracool dwarf discoveries confirmed by Euclid spectroscopy, all located in the EDF-S field (Section 6). These discoveries span spectral types M6–L2. Guided by the Euclid spectroscopic validation of the USMILE models, we further identify a set of high-quality M6–L9 candidates that lack high-S/N Euclid spectroscopy (Section 7), shown as blue stars.