  Rare-earth chalcogenide compounds $\mathrm{ARECh_2}$ (A = alkali or
monovalent metal, RE = rare earth, Ch = O, S, Se, Te) are a large family of
quantum spin liquid (QSL) candidate materials. $\mathrm{NaYbS_2}$ is a
representative member of the family. Several key issues on $\mathrm{NaYbS_2}$,
particularly how to determine the highly anisotropic spin Hamiltonian and
describe the magnetism at finite temperatures and the ground state, remain to
be addressed. In this paper, we conducted an in-depth and comprehensive study
on the magnetism of $\mathrm{NaYbS_2}$ from finite temperatures to the ground
state. Firstly, we successfully detected three crystalline electric field (CEF)
excitation energy levels using low-temperature Raman scattering technique.
Combining them with the CEF theory and magnetization data, we worked out the
CEF parameters, CEF energy levels, and CEF wavefunctions. We further determined
a characteristic temperature of $\sim$40 K, above which the magnetism is
dominated by CEF excitations while below which the spin-exchange interactions
play a main role. The characteristic temperature has been confirmed by the
temperature-dependent electron spin resonance (ESR) linewidth. Low-temperature
ESR experiments on the dilute magnetic doped crystal of
$\mathrm{NaYb_{0.1}Lu_{0.9}S_2}$ further helped us to determine the accurate
$g$-factor. Next, we quantitatively obtained the spin-exchange interactions in
the spin Hamiltonian by consistently simulating the magnetization and specific
heat data. Finally, the above studies allow us to explore the ground state
magnetism of $\mathrm{NaYbS_2}$ by using the density matrix renormalization
group. We combined numerical calculations and experimental results to
demonstrate that the ground state of $\mathrm{NaYbS_2}$ is a Dirac-like QSL.
