Both of the answers you got are partly describing real rules, and the reason they conflict is that these rules are set state by state and often city by city.
Many states have an owner-builder provision that lets the owner of a property pull permits and act as their own general contractor on their own home, sometimes with limits on resale timing or on how many properties you can do. Separately, most states license the specialty trades, electrical and plumbing especially, and those licenses apply to whoever does the work regardless of who owns the building. So an owner-builder permit doesn't usually let you or an unlicensed helper rewire a panel. Your building department can tell you exactly where those lines sit locally, and a license question with real money behind it is worth ten minutes with an attorney in your state.
On what a general contractor charges for: they hold the contract, schedule the trades in the right order, order and take delivery of materials, handle inspections, and fix it when something is wrong. That usually shows up as either a percentage on top of costs, commonly in the 10 to 20 percent range, or a single fixed price with the markup buried inside.
The piece people miss is insurance. If an uninsured crew member gets hurt on your job, that can land on you as the property owner, and your ordinary homeowners policy may not cover a gut renovation at all. Ask for a certificate of insurance from every crew and ask your insurer in writing whether the policy stays in force during vacant renovation.
