It's still a valid question. You are right, one has to bootstrap the root servers. There are a few ways to do this. Assuming one had working DNS server at some point in the past they can
dig @e.root-servers.net +nocookie +tries=4 +retry=4 +time=8 . ns | grep -Ev "^;|^$" > /etc/unbound/named_hints.tmp # sanity check this
and then do sanity checks on the output prior to loading it as hints in Unbound DNS. The 3K or so root servers are Anycast IP addresses and rarely change so this file will not be stagnant for a very long time thus making thumb drives a valid way to store and transfer this file.