  Topological insulators, along with Chern insulators and Quantum Hall
insulator phases, are considered as paradigms for symmetry protected
topological phases of matter. This article reports the experimental realization
of the time-reversal invariant helical edge-modes in bilayer graphene/monolayer
WSe$_2$-based heterostructures -- a phase generally considered as a precursor
to the field of generic topological insulators. Our observation of this elusive
phase depended crucially on our ability to create mesoscopic devices comprising
both a moir\'e superlattice potential and strong spin-orbit coupling; this
resulted in materials whose electronic band structure could be tuned from
trivial to topological by an external displacement field. We find that the
topological phase is characterized by a bulk bandgap and by helical edge-modes
with electrical conductance quantized exactly to $2e^2/h$ in zero external
magnetic field. We put the helical edge-modes on firm grounds through
supporting experiments, including the verification of predictions of the
Landauer-B$\mathrm{\ddot{u}}$ttiker model for quantum transport in
multi-terminal mesoscopic devices. Our non-local transport properties
measurements show that the helical edge-modes are dissipationless and
equilibrate at the contact probes. We achieved the tunability of the different
topological phases with electric and magnetic fields, which allowed us to
achieve topological phase transitions between trivial and multiple, distinct
topological phases. We also present results of a theoretical study of a
realistic model which, in addition to replicating our experimental results,
explains the origin of the topological insulating bulk and helical edge-modes.
Our experimental and theoretical results establish a viable route to realizing
the time-reversal invariant $\mathbb{Z}_2$ topological phase of matter.
