You are slightly correct.
Domain registration/moves can take anything from 1 hour to a few days to propagate the first time the domain is registered/moved and pointed to a name server. However, once a name server exists, the TTL can be anything from 1 hour onwards.
Since the DNS for amazon already exists at the top level, you can add as many subdomains as you want and they will appear almost instantly depending where in the world you are. There is really no magic here, since the DNS server already knows which name server to query.
Also, keep in mind that most DNS queries are cached after the first query, so often a client machine won't see a DNS entry because it was not available when queried the first time, and it normally resets the cache every 24 hours, or you can force the cache to be flushed on most major operating systems.