  We study the expected spin misalignments of merging binary black holes (BHs)
formed in isolation by combining state-of-the-art population-synthesis models
with efficient post-Newtonian evolutions, thus tracking sources from stellar
formation to gravitational-wave detection. We present extensive predictions of
the properties of sources detectable by both current and future
interferometers. We account for the fact that detectors are more sensitive to
spinning BH binaries with suitable spin orientations and find that this
significantly impacts the population of sources detectable by LIGO, while this
is not the case for 3rd-generation detectors. We find that three formation
pathways, differentiated by the order of core collapse and common-envelope
phases, dominate the observed population, and that their relative importance
critically depends on the recoils imparted to BHs at birth. Our models suggest
that measurements of the "effective-spin" parameter $\chi_{\rm eff}$ will allow
for powerful constraints. For instance, we find that the role of spin
magnitudes and spin directions in $\chi_{\rm eff}$ can be largely disentangled,
and that the symmetry of the effective-spin distribution is a robust indicator
of the binary's formation history. Our predictions for individual spin
directions and their precessional morphologies confirm and extend early toy
models, while exploring substantially more realistic and broader sets of
initial conditions. Our main conclusion is that specific subpopulations of BH
binaries will exhibit distinctive precessional dynamics: these classes include
(but are not limited to) sources where stellar tidal interactions act on
sufficiently short timescales, and massive binaries produced in pulsational
pair-instability supernovae. Measurements of BH spin orientations have enormous
potential to constrain specific evolutionary processes in the lives of massive
binary stars.
