  Understanding the microscopic mechanisms of thermalization in closed quantum
systems is among the key challenges in modern quantum many-body physics. We
demonstrate a method to probe local thermalization in a large-scale many-body
system by exploiting its inherent disorder, and use this to uncover the
thermalization mechanisms in a three-dimensional, dipolar-interacting spin
system with tunable interactions. Utilizing advanced Hamiltonian engineering
techniques to explore a range of spin Hamiltonians, we observe a striking
change in the characteristic shape and timescale of local correlation decay as
we vary the engineered exchange anisotropy. We show that these observations
originate from the system's intrinsic many-body dynamics and reveal the
signatures of conservation laws within localized clusters of spins, which do
not readily manifest using global probes. Our method provides an exquisite lens
into the tunable nature of local thermalization dynamics, and enables detailed
studies of scrambling, thermalization and hydrodynamics in strongly-interacting
quantum systems.
