GoKawiilAs with most other social networks, your username must be unique. That means no matter how much you might want to be known as GlitterQueen4Eva, if someone else snatches that title first, you'll need to come up with an alternative. The only way you might be lucky enough to grab an already-taken username is if that person opts to change what they have reserved, which can happen. Once a claimed handle has been changed or deleted, that name goes back up for grabs after 14 days. Meta hasn't specified how many times a person will be allowed to change their handle, but a limit will be applied at some point.
In the above menus, if you don't want to create a new ID for yourself, you'll see an option to use your same username from Facebook or Instagram. Assuming nobody else has already snagged it, you can lock that name down. You may want to go with an original name for WhatsApp if privacy is your goal for a username.
In more practical terms, a username must be between three and 35 characters. They can contain lowercase letters, numbers, underscores and periods. You won't be allowed to have a username of all numbers, and "restricted words or phrases" also won't be permitted.