Maintenance for the week of March 16:
• PC/Mac: No maintenance – March 16
• ESO Store and Account System for maintenance – March 18, 6:00AM EDT (10:00 UTC) - 4:00PM EDT (20:00 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+.