Alphabet Inc.’s Google and Samsung Electronics Co. unveiled a joint push into the mixed-reality market, introducing a new operating system and headset in a bid to challenge devices from Apple Inc. and Meta Platforms Inc.In what they called a collaboration as “one team,” the two companies announced a version of Google’s Android software for XR — shorthand for extended reality, which refers to a range of virtual- and augmented-reality technologies. They also showed off a Samsung-built headset code...