I've been looking through many videos online and can't find a simple explanation as to who market makers are and what exactly they do. 

If I'm using Etrade to buy a stock, does Etrade then submit the order to Etrade's market maker? Or Are there a bunch of market makers assigned to just a few stocks? I'm lacking a concept of how the market makers, brokers, and traders all fit together. Are market makers independent contractors who are hired by brokers, and/or are they investment firms? I've watched basically all there is to watch on youtube about this, and would very much appreciate an elementary school level explanation as to these questions I posed. 

 

Thanks! :)