  We present analysis of the light curves (LCs) of 77 hydrogen-poor
superluminous supernovae (SLSNe-I) discovered during the Zwicky Transient
Facility Phase-I operation. We find that the majority (67\%) of the sample can
be fit equally well by both magnetar and ejecta-circumstellar medium (CSM)
interaction plus $^{56}$Ni decay models. This implies that LCs alone can not
unambiguously constrain the physical power sources for a SLSN-I. However, 23\%
of the sample show inverted V-shape, steep declining LCs or features of long
rise and fast post-peak decay, which are better described by the CSM+Ni model.
The remaining 10\% of the sample favor the magnetar model. Moreover, our
analysis shows that the LC undulations are quite common, with a fraction of
$18-44\% $ in our gold sample. Among those strongly undulating events, about
62\% of them are found to be CSM-favored, implying that the undulations tend to
occur in the CSM-favored events. Undulations show a wide range in energy and
duration, with median values (and 1$\sigma$ errors) being as
$1.7\%^{+1.5\%}_{-0.7\%}\,\rm E_{\rm rad,total}$ and
$28.8^{+14.4}_{-9.1}$\,days, respectively. Our analysis of the undulation time
scales suggests that intrinsic temporal variations of the central engine can
explain half of the undulating events, while CSM interaction can account for
the majority of the sample. Finally, all of the well-observed He-rich SLSNe-Ib
have either strongly undulating LCs or the LCs are much better fit by the
CSM+Ni model. These observations imply that their progenitor stars have not had
enough time to lose all of the He-envelopes before supernova explosions, and
H-poor CSM are likely to present in these events.
