I don't understand who eats at McDonalds or shops at Walmart

They can say that of you and me.

I abjure all of the American fast-food chains and never used Walmart even when it had UK branches by having taken over an existing, British supermarket company; but clearly MacD and Walmart must be doing something right for so many people.

I will not use Amazon either, because I do not want to be part of my country's shops, and ultimately ranges of goods, being destroyed by a few giant American and Chinese on-line retailers. If I do want doorstep grocery deliveries, our supermarkets operate such services.

It's their choice even if you don't understand that.

I stopped eating McDonald's and I only go to Walmart if they have stuff I need but I like to shop online unless it's something local I need.

No way. Both places are masters of illusion.
Masters of marketing.
Masters of manipulation.

Once you have mastered those things, you will always make money.

Now you know. 🙂