  We study the topological bands in twisted bilayer transition metal
dichalcogenides in an external magnetic field. We first focus on a paradigmatic
model of WSe$_2$, which can be described in an adiabatic approximation as
particles moving in a periodic potential and an emergent periodic magnetic
field with nonzero average. We understand the magnetic-field dependent spectra
of WSe$_2$ based on the point net zero flux, at which the external field
cancels the average emergent field. At this point, the band structure
interpolates between the tightly-bound and nearly-free (weak periodic
potential) paradigms as the twist angle increases. For small twist angles, the
energy levels in a magnetic field mirror the Hofstadter butterfly of the
Haldane model. For larger twist angles, the isolated Chern band at zero flux
evolves from nearly-free bands at the point of net zero flux. We also apply our
framework to a realistic model of twisted bilayer MoTe$_2$, which has recently
been suggested to feature higher Landau level analogs. We show that at negative
unit flux per unit cell, the bands exhibit remarkable similarity to a
backfolded parabolic dispersion, even though the adiabatic approximation is
inapplicable. This backfolded parabolic dispersion naturally explains the
similarity of the Chern bands at zero applied flux to the two lowest Landau
levels, offering a simple picture supporting the emergence of non-Abelian
states in twisted bilayer MoTe$_2$. We propose the study of magnetic field
dependent band structures as a versatile method to investigate the nature of
topological bands and identify Landau level analogs.
