The patty in a McDonald's hamburger is a "synthetic meat," which uses a variety of different meats that are then processed, spliced, etc. to make a new patty. "Synthetic meat" is generally pork, chicken, beef-based, plus starch. "Synthetic meat" is cheap, so it is widely used in fast food, canned food, ham and sausages, and other foods.

Another netizen replied that it is 100% beef, I really think that person is retarded when I read that. Do you know how expensive beef is? McDonald's is twenty-something dollars a hamburger and you want beef? You think that's a charity?

And McDonald's ads never mention that theirs is 100% pure beef, they just say "beefburger". A "beef burger" can be called a "beef burger" even if it has only 1% beef in it, and the ads are just picking words.