Different markets entirely—I can't walk into amazon, and I don't order online from Walmart.

You can order online from Walmart:

https://www.walmart.com/

Amazon can ship it to a location near you.

Again, different markets, because I'm not going to do either of those things—if I'm ordering online amazon has better selection, and if I want to walk somewhere to pick something up I'm not going to wait for shipping.

Are you saying that Amazon is a successful monopoly, or that Amazon is even with massive expenses still not a full monopoly?

Walmart competes with Amazon.