I will copy/paste some examples from the docs here:
topo = topology(grid)
# create boundary relation mapping
# 2-faces to 0-faces (i.e. vertices)
∂ = Boundary{2,0}(topo)
# list of vertices for first face
∂(1)
# boundary relation from faces (dim=2) to edges (dim=1)
∂ = Boundary{2,1}(topo)
# show boundary of first n-gon
∂(1)
Please check the docstrings of Adjacency, Boundary and Coboundary.