Good point on "http" vs. "https", which refers to the protocol to be used when accessing services running on a host reachable by a certain domain name, and which has nothing to do with the domain name registration or ownership.
Domain name, in this case ''ivdp" is certainly unique but only within a specific, so called gTLD (generic Top Level Domain) namespace. So, you can buy ivdp.com, but that still leaves "ivdp" name available in other gTLDs - for example, .net, .org, etc. Each of these (.net, .org, etc) is a separate gTLD registry (a database where the name records are kept) independent of each other. You can buy one or all - your choice. It is common for large companies to buy many or all of them to prevent leakage of their goodwill or brand value. For example, check out 'google' with .net, .org, .is, .to, etc...
Also, if you want to find out who owns a particular domain, a wide range of tools is available, for example, the official
https://lookup.icann.org/en/lookup or (a better one :)?) -
https://www.domain.com/whois/whois/?search=ivdp.com . However, most of the personally identifiable information will be redacted from the records. You may find out the registrar - a proxy company which handles registration on behalf of the registrant/owner. I can see that "ivdp.com" registration was requested by fabulous.com, and looking through the rest of the records for this registration, I spot a strangely named fallback DNS server - THIS-DOMAIN.NAME-IS-FOR-SALE.COM It's not hard to guess at this point that you can buy this domain. :)
Btw, in case you will be trying to lookup the Portugal version, the lookup service may not be available for all gTLDs. Specifically the .pt gTLD is most certainly managed by the country of Portugal and has its own services to access information in the registry. So, I don't think you will be able to look up "ivdp.pt" on the icann.org or domain.com.
On a related topic, it is not actually possible to buy a domain name - it's only possible to rent it. When you "buy" a domain name, you have to confirm your ownership every year or so though some registrars may have auto-renew services to extend the "rent" automatically for you. The official max "ownership" time (according to ICANN) is set to 10 years I think. The "fun" part here is that occasionally, the auto-renew doesn't happen, and your domain name registration expires at which point anyone can try to purchase it. There are companies that do that intentionally - looking for expired records, buying them, and then asking ransom from forgetful owners... I've seen that happen (luckily not to me). :)
Cheers,