  When discovered, SN~2017egm was the closest (redshift $z=0.03$) hydrogen-poor
superluminous supernova (SLSN-I) and a rare case that exploded in a massive and
metal-rich galaxy. Thus, it has since been extensively observed and studied. We
report spectroscopic data showing strong emission at around He~I
$\lambda$10,830 and four He~I absorption lines in the optical. Consequently, we
classify SN~2017egm as a member of an emerging population of helium-rich
SLSNe-I (i.e., SLSNe-Ib). We also present our late-time photometric
observations. By combining them with archival data, we analyze high-cadence
ultra-violet, optical, and near-infrared light curves spanning from early
pre-peak ($\sim -20\,d$) to late phases ($\sim +300\,d$). We obtain its most
complete bolometric light curve, in which multiple bumps are identified. None
of the previously proposed models can satisfactorily explain all main
light-curve features, while multiple interactions between the ejecta and
circumstellar material (CSM) may explain the undulating features. The prominent
infrared excess with a blackbody luminosity of $10^7$--$10^8\,L_{sun}$ detected
in SN~2017egm could originate from the emission of either an echo of a
pre-existing dust shell, or newly-formed dust, offering an additional piece of
evidence supporting the ejecta-CSM interaction model. Moreover, our analysis of
deep $Chandra$ observations yields the tightest-ever constraint on the X-ray
emission of an SLSN-I, amounting to an X-ray-to-optical luminosity ratio
$\lesssim 10^{-3}$ at late phases ($\sim100-200\,d$), which could help explore
its close environment and central engine.
