The patent US 5782079 A is filed on February 1997. So it's been more than 20 years since it first patented. So this means this patent is no longer protected by the patent. So, can this be manufactured by someone who's not the patent holder and sell the product worldwide? and What's the legal status of the patents after 20 years. I need to know this for educational purposes.
For patents applied for after June 8, 1995, the expiration date is 20 years from the priority date. You are correct that means US5782079A has expired. I checked the US Public Pair to see if there was a patent term extension and there wasn't. According to Pair, the patent expired for non-payment of fees on 08-16-2010. US patents only apply to the US so you have to make sure there aren't international patent equivalents although those usually share priority and expiration dates. In principal you should be able to sell the device as described in the patent. You need to be aware that there can be other relevant patents. If you look at the cited and similar documents link for this patent you can see other patent citations many of which are newer.