Is "off-label" to mean generic?
Off-label and generic are two different terms. They are not related.
When a drug is approved for use by the FDA, based on successful large-scale clinical trials, it is approved only for the use specified in the clinical trial. The FDA requires that the use be printed on the drug container label, and that the use be spelled out in detail on the printed package insert. That's the 'on-label' use. Under US law, the manufacturer of the drug cannot advertise or promote any use of the drug other than on-label uses. However, physicians are free to prescribe a drug for other 'off-label' uses. For a drug like ivermectin, using it to treat Covid-19 is off-label, because it hasn't been tested through a large-scale clinical trial. Just because a physician is free to prescribe off-label use of a drug doesn't make him or her free from medical malpractice lawsuits.
I should add that medical insurance companies will probably not pay for off-label use of expensive drugs. However, ivermectin is generic (see below) and cheap, and it gets complicated because ivermectin has been approved for human use only to treat parasitic worms.
If a new drug is patented by the manufacturer, as most are, they are protected against competitors selling a copy of the drug for 16 years. (Is this still 16 years, or has patent protection been extended?) After the patent expires, competitors are free to sell copies of the drug – the 'generic' form of the drug.