  Small-twist-angle transition metal dichalcogenide (TMD) heterobilayers
develop isolated flat moir\'e bands that are approximately described by
triangular lattice generalized Hubbard models [PhysRevLett.121.026402]. In this
article we explore the metallic and insulating states that appear under
different control conditions at a density of one-electron per moir\'e period,
and the transitions between them. By combining fully self-consistent
Hartree-Fock theory calculations with strong-coupling expansions around the
atomic limit, we identify four different magnetic states and one nonmagnetic
state near the model phase diagram's metal-insulator phase-transition line.
Ferromagnetic insulating states, stabilized by non-local direct exchange
interactions, are surprisingly prominent.
