  JWST observations have opened a new chapter in studies of supermassive black
holes (SMBHs), stimulating discussion of two puzzles: the abundance of SMBHs in
the early Universe and the fraction of dual AGNs. In this paper we argue that
the answers to these puzzles may be linked to an interpretation of the data on
the nHz gravitational wave (GWs) discovered by NANOGrav and other Pulsar Timing
Arrays (PTAs) in terms of SMBH binaries losing energy by interactions with
their environments as well as by GW emission. According to this interpretation,
the SMBHs in low-$z$ AGNs are the tip of the iceberg of the local SMBH
population, which are mainly in inactive galaxies. This interpretation would
favour the observability of GW signals from BH binaries in LISA and deciHz GW
detectors.
