STP dealers are generally small and medium-sized companies, so they can't make their own quotations, and they need bank quotations. The spread is floating, because the company's strength is limited and the losses it can bear are limited, so when the market fluctuates greatly, the spread will also expand, which can reduce the risks borne by traders.
ECN dealers are usually very small companies, which implement a price matching mechanism and submit their clients' positions to the open interbank market, similar to trading in the stock market. They follow the international market completely and there is no price difference. There is a handling fee for every transaction. If the transaction volume is large, the handling fee is favorable.