The block is returned to ARIN, not to IANA. ARIN will of course keep it and - I suppose very soon - re-delegate it. TFA talks about the exhaustion of blocks at IANA (though it wrongly names ARIN there). The RIRs like ARIN still have some blocks to distribute for the next months. IPv4 exhaustion is slowly tumbling downwards through the registries to the ISPs and then companies, webhosters, etc.

Not randomly. See Default Address Selection for Internet Protocol version 6 (IPv6).