There's no cross platform play, except for Faction War. And thank god for that. Ubisoft are able to push patches on PC at any time they want to test things, and they have been doing so with Siege and Steep before deploying updates to consoles, because on consoles, MS and Sony need to spend weeks on verifying the update.
Cross platform play would put a stop to PC updates whenever Ubisoft feel like it, and would leave bugs in the game for longer before people are able to catch them.
Now if they want to release a patch on PC to test out changes, they can do so immediately, and if people find a serious bug 5 minutes later, they can push a new update as soon as it's fixed. For cross platform play, they'd need keep consoles and PC patched to the same versions, and they'd need to wait a minimum of 2 weeks between patches, and if people find a serious bug 5 minutes later, they'd need 2 weeks before they could push a fix.