Please explain in detail the principle used in hot air balloons

The buoyancy force on the hot air balloon = the weight of the air it displaces. Since the gas in the hot air balloon is hot air and is relatively thin, its weight is much smaller than the weight of the air displaced. The weight of the hot air balloon itself The weight (load) is not large, and the weight of the displaced air is greater than the sum of the weight of the hot air and the hot air balloon, so the resultant force on it is upward, so it flies up