  The boundary Green's function (bGF) approach has been established as a
powerful theoretical technique for computing the transport properties of
tunnel-coupled hybrid nanowire devices. Such nanowires may exhibit
topologically nontrivial superconducting phases with Majorana bound states at
their boundaries. We introduce a general method for computing the bGF of
spinful multi-channel lattice models for such Majorana nanowires, where the bGF
is expressed in terms of the roots of a secular polynomial evaluated in complex
momentum space. In many cases, those roots, and thus the bGF, can be accurately
described by simple analytical expressions, while otherwise our approach allows
for the numerically efficient evaluation of bGFs. We show that from the
behavior of the roots, many physical quantities of key interest can be
inferred, e.g., the value of bulk topological invariants, the energy dependence
of the local density of states, or the spatial decay of subgap excitations. We
apply the method to single- and two-channel nanowires of symmetry class D or
DIII. In addition, we study the spectral properties of multi-terminal Josephson
junctions made out of such Majorana nanowires.
