A natural tank has no filters (or a sponge filter), maybe a heater and lights. It would be 10 gallon tank with lots of live plants an smooth stones. A snail/shrimp or two and a betta. It would be like the wild except the cleaning periods. It would make your betta less stressed and would make him calm and happy. That is a natural tank.
A man-made tank would be 2.5 gallons or more with a filter, heating, and lights. It may have live plants, but not much. It would more likely to have plastic and silk plants. It might even have a bare bottom. A more stressed and aggressive betta may form here, but maybe not.
A horrible tank would be a small bowl or a cup with nothing else but water. That is boring.
I hope these ideas will help you. Remember to use water conditioner, age the water, and rinse all the stuff.