Because crossplay hasn't been around for a long time, developers are still struggling to implement the feature with zero problems. More often than not, it takes studios several patches to get crossplay to work properly, making it more experimental at the moment, rather than an exact science. This might apply to Halo Infinite, the free-to-play multiplayer mode of which was just released on November 15.
As part of Microsoft's 20th-anniversary celebration for the Xbox console, 343 decided to drop Halo Infinite just under a month ahead of its intended launch date. Because this is technically still the game's "beta" period, Halo Infinite isn't perfect. For example, Halo Infinite servers have gone down now and then. Meanwhile, the Battle Pass was grindy as hell and 343 has promised to fix it. In addition to these issues, Halo Infinite is also confusing some players because it's available to play on the Xbox Series S/X, Xbox One, and PC.
If you'd like to know more about how cross-play and cross-progression works in Halo Infinite, read on.