good explanation, but how do expect a beginner like the OP to understand your explanation?
Try Googling your broker name + payment for order flow. Many brokers do not send the orders directly to the ECNs - instead they get paid by HFTs like Citadel which takes the bid-ask spread and uses non-marketable orders as insurance to use when price rolls over. The HFT can end up adding more in costs than your brokers commission - for real.
Some brokers like IB, Lightspeed, TradeStation, & Fidelity provide direct market acess (DMA) which lets you choose to route directly to the ECN's you choose. This way you are taking the middleman out, reducing your costs and getting faster fills.
* Fidelty's DMA does not have a route to the IEX *
* These are the brokers I am aware of that have DMA, I am not implying they are the only ones - do your own DD.