there is no "rarity" with domain names. They are simply for sale. You can go buy any domain you want if it's not taken (and unless it's .gov or .mil, I believe those are restricted)

"top level" just refers to that part of a website address. Think of it like a home address. "Road" or "Lane" etc would be the top level domain, then the road name would be the next level, and then the house number would specify exactly where on that particular road you want to go.

Top level as in it's the top/beginning of the domain. (reading right to left separated by periods)

Domains have different parts: subdomain.rootzonedomain.topleveldomain

So using the example: google.com

Google owns the 'google' portion of the domain, called the root zone domain. They pay VeriSign Global Registry Services (the owner of the 'com' top level domain) to keep the fully qualified domain name 'google.com' registered to google inc. Google can then add anything they want to the beginning of the domain such as keep.google.com. These are called sub-domains. They could be part of and used by the company itself or even rented out by google the same way google pays VeriSign for their 'com' registration.

For example: No-ip is a service that will freely rent subdomains of their no-ip.com domain, mainly so you can easily reach your self-hosted services without needing to remember your home IP. You could host lemmy.no-ip.com if you wanted to.

The owners of sought after tlds like 'com' and 'net' set the prices higher than some others as there is more demand. Because of this many self-hosters amd smaller companies just use cheaper tlds like 'ml'. The extra cost isn't really worth it.