Independent contractors are not required to form business structures for their work. If they wish, they can continue working as a contractor for as long as they want, and they will be sole-proprietors by default.
However, there are plenty of independent contractors out there who choose to form an LLC or S-corp. You may be wondering why they are doing it and if you should also follow suit. If this is your case, then you have come to the right page. Here, you will know more about LLC and S-corp and whether you should go for any of it as an independent contractor.