  Binary black holes (BBHs) are thought to form in different environments,
including the galactic field and (globular, nuclear, young and open) star
clusters. Here, we propose a method to estimate the fingerprints of the main
BBH formation channels associated with these different environments. We show
that the metallicity distribution of galaxies in the local Universe along with
the relative amount of mergers forming in the field or in star clusters
determine the main properties of the BBH population. Our fiducial model
predicts that the heaviest merger to date, GW170729, originated from a
progenitor that underwent 2--3 merger events in a dense star cluster, possibly
a galactic nucleus. The model predicts that at least one merger remnant out of
100 BBH mergers in the local Universe has mass $90 < M_{\rm rem}/ {\rm
~M}_\odot \leq{} 110$, and one in a thousand can reach a mass as large as
$M_{\rm rem} \gtrsim 250$ M$_\odot$. Such massive black holes would bridge the
gap between stellar-mass and intermediate-mass black holes. The relative number
of low- and high-mass BBHs can help us unravelling the fingerprints of
different formation channels. Based on the assumptions of our model, we expect
that isolated binaries are the main channel of BBH merger formation if $\sim
70\%$ of the whole BBH population has remnants masses $<50$ M$_\odot$, whereas
$\gtrsim{}6$\% of remnants with masses $>75$ M$_\odot$ point to a significant
sub-population of dynamically formed BBH binaries.
