Two words: lacquer & airbrushing.
When I worked in graphic arts in the 70s, we did a lot of Carls’ Jr. hamburger shots for ads. The photographer would spray clear lacquer on the the burger to make it look fresh and shiny. The lettuce was fake, the real stuff would wilt under the hot lights.
After the final shot was picked it was heavily airbrushed to make the burger look like a million whith shiny bright areas on the cheese, meat, lettuce and bun.
I’m sure it s all done in Photoshop now days.
I used to edit an employee magazine for a large food processing company. Food images may be among the most difficult to shoot to make the food seem appetizing. In lots of local restaurant commercials the colors come out all wrong and drive customers away!
One of my worst assignments was trying to cook fried eggs sunny-side up so they looked appealing. Cooking them that way is a real challenge; you have to use ultra-low heat to get the whites to set without discoloring the yolks. If people routinely cook breakfast eggs that way easily I'd love to know how it's done!