In reality, you get both. People don’t shop at Costco OR Amazon. People primarily go to Costco for food (then stumble on everything else), Amazon has struggled over and over to get their grocery business to catch on. They’re just the best source of the everything else part.

These are only comparable in an academic business model comparison, in reality, these are different retailers selling different things and consumers behave differently depending on context of what they’re buying. A lot of people want low cost on food, meanwhile, they’ll spend superficially on disposable plastic junk with very little practical value. I’m taking about the American consumer specifically when I say everyone.