Maintenance for the week of December 8:
• [COMPLETE] ESO Store and Account System for maintenance – December 10, 9:00AM EST (14:00 UTC) - 12:00PM EST (17:00 UTC)
Like every other expansion for one year or so following their release, they are a separate thing. You need to buy it, along with the base game, to play it. After about a year or so, they'll be antiquated and turned into "DLC", which means you can buy it with crowns, gift/sell to other players and access it with ESO+.