  Graphene matter in a strong magnetic field, realizing one-dimensional quantum
Hall channels, provides a unique platform for studying electron interference.
Here, using the Landauer-B\"uttiker formalism along with the tight-binding
model, we investigate the quantum Hall (QH) effects in unipolar and bipolar
monolayer-bilayer graphene (MLG-BLG) junctions. We find that a Hall bar made of
an armchair MLG-BLG junction in the bipolar regime results in valley-polarized
edge-channel interferences and can operate a fully tunable Mach-Zehnder (MZ)
interferometer device. Investigation of the bar-width and magnetic-field
dependence of the conductance oscillations shows that the MZ interference in
such structures can be drastically affected by the type of (zigzag) edge
termination of the second layer in the BLG region [composed of vertical dimer
or non-dimer atoms]. Our findings reveal that both interfaces exhibit a double
set of Aharonov-Bohm interferences, with the one between two oppositely
valley-polarized edge channels dominating and causing a large-amplitude
conductance oscillation ranging from 0 to $ 2 e^2 / h$. We explain and analyze
our findings by analytically solving the Dirac-Weyl equation for a gated
semi-infinite MLG-BLG junction.
