> What is the process or method this spammers use to post as a comment?
they usually make posts (manually or automatically) as normal users, writing "rubbish" or advertising some products/services, optionally posting some links
> can spammers post a comment without registering as a User?
if you open your website to anonymous users (permission "adding comments" set to "Anybody"), yes
> What feature I need to enable so that every user who registers need to be approved by Admins?
Admin --> Spam -->
--> Enable moderation (approval) of users (checked)--> All new users must be approved (checked)