Console players and PC players will be kept separate.
Rainbow Six Siege - Nighthaven Labs map

On December 6, Ubisoft will release an update for Rainbow Six Siege that will, among other things, offer players a chance to indulge in both crossplay and cross-progression features. These features will arrive as part of the upcoming seasonal update called Operation Solar Raid. This update will feature other new content such as a new map, playable character, and more.

The reveal came first from Eurogamer, who says that console players on Xbox and PlayStation will get to all play together while PC gamers will be in a separate pool with Stadia and Luna players. That is to say that PC players will probably still just mostly see other PC players.

On the 6th, console players will get access to their progress, currencies, and other items across all shared platforms just by logging into their accounts. They can also team up with friends on other consoles through the use of Ubisoft Connect.

For those curious, you can disable crossplay if you want to. I cannot imagine why you would really want to though since you'll just be playing with other console players already and won't have to worry about any PC players, but the choice is still there if you want it.

Also on the 6th will be the addition of Operation Solar Raid, which will add in Defender Solis, a "medium speed, medium health" Operator. They will be armed with a P90 SMG or the ITA 12L shotgun for a primary weapon and an SMG-11 for the secondary. Solis has a SPEC-IO Electro-Sensor gadget that can find and identify Attacker devices such as drones and breaching devices. She will be able to "interact with the gadget overlay and trigger a cluster scan."

Her unique gadget is the SPEC-IO Electro Sensor, an augmented reality headset that enables her to spot and identify the Attackers' electronics gadgets. It functions on a charge that will slowly drain when she's using it, and refill when she's not. She can also spend the device's charge to cluster scan all the gadgets in her sight and tag them for her allies to see, in addition to simply yellow-pinging them or verbally calling them out.

Solis is a powerful counter for any Attacker whose unique gadget is powered by electricity, allowing her to detect intel-gatherers like Zero, roamer-hunters like Jackal, stealth Operators like Nøkk, and pretty much everyone trying to breach a wall. Plus, every Attacker has a drone, and Solis doesn't just spot the drone you're controlling, she spots the phone you're using to control it. Solis' gadget is a powerful tool for revealing what the Attackers are up to so that she and her teammates can seize the upper hand.​
The new map is called Nighthaven Labs and will enable for players to carry out some "creative strategies." There will also be a new Ranked 2.0 system, better anti-cheat, plus a new Battle Pass and progression system.

This high-tech facility was first seen in the Operation Brutal Swarm anime trailer as the setting for the showdown between Grim and Nøkk, and now you can skulk through its halls in search of your foes. Attackers have a lot of exterior room to maneuver, from the stylishly landscaped main entry to the heavily laden helipad area. There are even a few breakable exterior walls that can allow access directly to some bomb sites, so Defenders will have to plan for how to keep the invaders at bay. Inside, there are multiple single-story staircases that allow transit from the basement to the first floor and the first to the second, but no stairs that allow a straight shot up through both levels, so all players will have to learn the layout if they want maximum mobility.