ReCore has started downloading via the Windows Store now but you can't play it just yet. The download is available via an update in the Windows Store and comes in at a size of 9.2GB. This looks to only be the first part of the download, with more to come ahead of the release.
ReCore will be one of the first Xbox Play Anywhere titles, allowing users to own it on Xbox and PC; as the official ReCore website puts it, you are Joule Adams, a survivor of a cataclysm:
You are Joule Adams, a young survivor of a terrible cataclysm who journeys from Earth to the desert world, “Far Eden” to build mankind’s new home. But when Joule wakes after centuries in cryo-sleep, nothing has gone according to plan… With her “Corebot” companion, Mack, Joule begins an adventure to unlock secrets from the past, defeat powerful foes, and bring new life and hope to Far Eden.
Trying to open the game after installing it just tells you that you're too early to play and will need to wait until the game releases in the local area.
The game should be available by September 13 if you live in America or Asia (except Japan that gets it the 15th). If you live in any other region you'll be able to launch the game by the 16th of September.