Perhaps in a technical sense that may be accurate. In practice, because it is frequently (if not always) advantageous for a company to have as few employees as possible, a company may try to call many of its employees "contractors" or "independent contractors." That ruse simply won't cut it and is where the complex employment law comes into play. So then, the company sets up a bunch of sub-corporations or captive corportations and tries the same thing. One might consider a corporation for house keeping, parking lot attendants, caffeteria staff, etc. (I have exaggerated a little, but not that much.) However, the laws have guidelines for determining whether a person is truly an independent contractor, a contracted employee of another corportation, an employee, etc. I have been each of those categories at one time or other.
So, back to your question of what contractors do. The simple answer is that they can do anything that an employee does, but may not have the same benefits or job security that an employee has. They can have fixed hours and get some, even most of the benefits of full-time employees, but that is not necessarily the case. On the other hand, they can be paid by the hour or by the job and totally free to set their own working conditions. Some of them never even show up on the company premisses. They simply work from home.
Again, if you are offered a job as a contractor and get more than a small amount of your income from that one position, say 20%, then you need good legal and professional advice. Some of those positions are very good, but some are full of potential potholes for your career. John