Your login credentials are stored in your Keychain. Each Keychain record consists of three elements - a Username, Password and Website URL.
The Username can consist of any printable character that the associated website will accept - including just numbers. Whilst email addresses are commonly used, they are not the only format of Username identifier that is accepted by your Keychain.
The same can be said of Passwords. Keychain can automatically generate a complex password - but this will not necessarily be accepted by some websites. Again, you can manually enter and printable characters that are accepted by the target website.