Age of Ultron has to walk a tricky line. It has to give us real, relatable characters (i.e., actual human beings), but it also has to show them doing unreal, totally amazing feats, like, you know, flying or destroying a skyscraper with their fists.
To get this to look just right, then, the movie used both conventional 35mm cameras as well as high-tech digital ones. The digital filming allowed the tech wizards over at Industrial Light and Magic (perhaps the most well-known special effects studio in the business) to work their, um, magic and superimpose computer-generated effects into an otherwise-conventional scene.
Let's go back to that touching moment when Black Widow calms down the Hulk after the movie's opening battle sequence. To pull that off, she puts her (comparatively) tiny pale hand on his massive green one. In this connection, reality is meeting a digital effect, but it has to look believable to carry the emotional truth of the moment. And thanks to this hybrid style of filmmaking, it does.