Domain name extensions?

I recommend checking out the Digital Literacy track on Treehouse as it covers a lot about what you are referring to. More specifically, check out this section which covers in detail how domains work as well as the protocols for accessing a webpage:

https://teamtreehouse.com/library/how-the-web-works

As for domain extensions, they are referred to as TLDs, or top level domains and include ones such as the familiar .com, .net, .org as well as several others. Each TLD, and domain associated with them exist independent of one another by default, meaning that mywebsite.com and mywebsite.org could be completely different websites, and have completely different owners as they both would have their own unique IP addresses that the domain routes to. You can think of a domain as essentially a subdomain of the top level domain. For example, my website is lovett.io, where ".io" is the TLD (top level domain) and "lovett" is the subdomain of that TLD. Someone else happens to own lovett.com and that website exists completely separate of my own, since it has a different TLD and in turn routes to a different IP address.

You are correct though in the case of companies like Facebook, Google, and even many smaller organizations where they will often buy many variations of their primary domain and redirect them to that primary domain. Often they do this to ensure that a user trying to access their site directly does not end up somewhere else by mistake if say, they make a small typo, or type in the wrong domain extension (Example, facebook.org instead of facebook.com). It also helps them to protect their company identity by attempting to ensure that nobody else can use any variation of their name for other purposes.

Again, I recommend checking out that track I mentioned as it will give you a more in-depth (and likely better explained) look at how it all works, but I hope this has helped clear things up a bit!