You only know what you are getting from McDonalds because you have bought it before, well what about your first time eating at McDonalds? You certainly did not know what you were getting then.
I suspect that for most people in the US their first meal at McDonalds was purchased by their parents. And they probably know quite a few people in real life that would endorse McDonalds.
That is a pretty much irrelevant point though, use any other example of buying a meal from somewhere that is not as common as McDonalds. On the point of endorsing, people can endorse an app as well so I am still not sure how that applies.