  Ancient Galactic Globular Clusters (GCs) have long fascinated astronomers due
to their intriguing multiple stellar populations characterized by variations in
light-element abundances. Among these clusters, Type-II GCs stand out as they
exhibit stars with large differences in heavy-element chemical abundances.
These enigmatic clusters, comprising approximately 17\% of analyzed GCs with
MPs, have been hypothesized to be the remnants of accreted dwarf galaxies. We
focus on one of the most debated Type~II GCs, NGC1851, to investigate its MPs
across a wide spatial range of up to 50 arcmin from the cluster center. By
using Gaia DR3 low-resolution XP spectra, we generate synthetic photometry to
perform a comprehensive analysis of the spatial distribution and kinematics of
the canonical and anomalous populations within this GC. By using appropriate
CMDs from the synthetic photometry in the BVI bands and in the $\rm f415^{25}$
band introduced in this work, we identify distinct stellar sequences associated
with different heavy-element chemical composition. Our results suggest that the
canonical and the anomalous populations reside both inside and outside the
tidal radius of NGC1851, up to a distance that exceeds by 3.5 times its tidal
radius. However, $\sim$80\% of stars outside the tidal radius are consistent
with belonging to the canonical population, emphasizing its dominance in the
cluster's outer regions. Remarkably, canonical stars exhibit a more circular
on-sky morphology, while the anomalous population displays an elliptical shape.
Furthermore, we delve into the kinematics of the multiple populations. Our
results reveal a flat/increasing velocity dispersion profile in the outer
regions and hints of a tangentially anisotropic motion in the outer regions,
indicating a preference for stars to escape on radial orbits.
