That has to do with Internet Assigned Numbers Authority (IANA), run by ICANN, who oversee IP address allocation and root zone management of DNS. DNS is the database/system that maps domain names to IP addresses, if you’re not aware.

No, tor has nothing to do with regular domain names.

With few exceptions, this is where the buck stops with the internet. If ICANN doesn't recognize you, "the world" can't associate your IP address to a domain name. Some "alt dns root zones" exist, but they're either rogue states, bad actors, or even in one case, a crypto grift.

Getting twitter.sucks is just leasing/buying a domain name, which is a lot more simple. Generally you pay for a set period, and have the option to renew it. I'm not sure what happens when a registrar closes down, other than it's likely someone else takes it over. The agreements linked below have sections describing what happens if a registrar closes down or gets shut down by ICANN for breaking the agreement... presumably someone else takes it over. There are surely other people who know more about this than I do.

You can start a new gTLD that nobody else has for $185,000, plus the cost of operating the registry. From what I understand it's basically a lease. As a registry, your job is to maintain a zone file for the TLD, and sell domains. (edit!) reading the contracts, you also pay them about $6,000 a quarter plus .25c for each registration for revenue over $50,000.

Here is the agreement ICANN has for people who start TLDs: https://www.icann.org/en/registry-agreements and it is fairly legal and arcane.

They have a FAQ about the gTLD program: https://newgtlds.icann.org/en/applicants/global-support/faqs/faqs-en

TLD, or top level domain, is the starting point of the domain name system. It's what comes at the end of the URL, and it's set up that way for very good reasons that I won't get into, because it would be a very long discussion.

You can think of the TLD a bit like a country, and the primary domain name, eg: "Twitter" as the company name. While "Twitter" is a valid business in the .com "country", it's unfounded in many others. So to push the analogy, it would be like McDonald's only having it's business in the USA, and someone in England starts a business in the UK, calling it McDonald's. This happens before McDonald's (USA) starts making any inroads into the UK to expand there. So the UK McDonald's owns the name and if McDonald's USA wants to expand to the UK, they can either buy the existing McDonald's company in the UK, or change their name for the UK. Same deal with domain names. If you own "Twitter" in the "country" (TLD) of .sucks, the owner of Twitter.com couldn't really do shit about you owning it.

There's the other obvious issue that if you're making a site like "twitter.sucks" you will have to be very careful not to infringe on their copy rights for things like their logo, etc. Especially if the basis of the site is to mock the .com version of the same.

DNS is a very interesting part of the internet. You can get rather deep into the structure and systems in use and how subdomains work and how complex they can be. For now, the only interesting quirk of the internet that I want to touch on is that there's technically a period at the end of every domain name (eg "twitter.com.") which is almost always omitted. That period represents the root of DNS, which is a thing that actually exists.