The act of domain registration and authoritative holder of DNS for that domain are separate things. When you have registered a domain normally some DNS is given authority or some means by which you can set the initial DNS to use it given to you.
Whois info can vary. Sometimes whois info is good, sometimes not so good, and of course, you may have to query different sources of register info...
In short, you can have a registered domain without anything on the Internet at all, and no DNS at all. (perhaps not too useful, but you may be planning something or just wanting to make sure you own the domain name)
Likewise you could operate a DNS that doesn't stand authoritative for any "zone" (including zones of a registered domain).
Generally speaking, most will obtain a registered domain and setup the intial DNS servers that will stand authoritative for DNS records of that domain.
whois is a "way" to query the databases of record for info about registered domains. However, because it contains "owner" info (names, addresses and such). Some believe that whois will eventually get shut down or changed so as to protect that data. We'll see.
When you query DNS A records, only DNS is used, whois is never involved. Again, the data in whois (multiple registrars) varies greatly.... can be hard to parse. And sometimes is woefully inaccurate, though I know the US whois servers are really trying to ensure their data stays accurate.
Again, think of DNS as a telephone book. Instead of name to phone number, it's name to IP address (A records).
Going the reverse is interesting too. When you go from IP to get the name (PTR records) the owner of those zones of authority have to do with the network (net block) and thus you may or may not have much control unless the net block owner of the DNS zone info delegates the serving of your network space to your DNS (for example).
In other words, when you own a domain, and setup DNS servres for the domain, we're talking about the A, MX, CNAMES, SRV etc... but *not* the PTR records (in-addr.arpa or ip6.arpa). So if you want to make changes for all you'll also need to work with your ISP or network provider... especially true if you don't actually "own" the IPs but have been given a set to use from your ISP (thus owned by your provider and not you).
Ok, I probably just given you more to think about... which may only add to your confusion.
Practice on unix.com. Do whois, do dns lookups. Do a whois on the IP returned for unix.com. There you'll see who owns the net block.
If you own your IPs (remember we're pretty much out of IPv4 blocks), then you can move them pretty easy from provider to provider. Which gives you a bit more flexibility. However, if the IP isn't as important as your registered name and you don't need the flexibility then you'll get something carved our of the netblock your ISP gives to you.
I can give you some examples, using talkfusion:
Whois-RWS
Those are the IPs they own.
Whois-RWS
These are IPs they also use, but come from their provider (datacenter), not really "owned" by them.
I don't know why I'm rambling on.. hopefully more help than harm... I'll post.. and go away...