There’s this topic here which covers part of backend changes after cross-play came in: Crossplay update soft region locked everyone - #127 by JoshNash-1223
Noteably: “The simple answer is, you are placed into the best match based on a variety of factors including player skill and ping. Getting more technical, it’s because the regions themselves no longer exist (at least not in the way players traditionally understand them), so there is no region to swap to.”