The company can make an offer to purchase any shares held by the public pretty much at any time and at any price that their board of directors thinks is reasonable.
The people who own those shares can either accept or decline the offer as they see fit.
In order for a company to be listed on an exchange, there are listing requirements having to do with a minimum number of shares outstanding and a minimum share price. If a company does not meet those requirements, then the stock will be delisted from the exchange. Maybe there are similar requirements for OTC markets, but I don't really know.
If a company is no longer traded, then those shares still held by the public are still held by the public, and those public owners still have all of their shareholder rights (such as voting rights and rights to receive dividends). Essentially they would be shareholders in a privately held company.
Shares in a privately held company can be bought and sold but I think that is harder to do (I think there are actually rules that are in place to help ensure that these transactions are "fair").
If a company is privately held, I don't think they're subject to the SEC regulations regarding releasing financial statements. But they can obviously release unaudited statements if they want to (as long as they're not misleading).
I don't think ticker symbols are "owned", per se. At least not on the exchanges. As far as I know, when a company goes public or does a stock spin-off, they can "claim" any ticker they want if it's available. I thought that's what happened when HWP spunoff A. I was surprised "A" was an available ticker symbol, but it was. Maybe rules are different for OTC stocks.
All of this may not be palatable to the shareholders that didn't want to sell at the offered price, but it was probably disclosed at the time that the company wanted to go private, so the risk that the stock would be delisted should have been a consideration. The only way to prevent a company from going private would be to buy enough of the stock to gain control of the board of directors, and most people can't afford to buy that much of a company. In the case of AAPL, it would be hundreds of billions of dollars.