so Strike missions are the main single player story mode?...
No. Strikes are the equivalent of "dungeons" in other MMOs, that will have difficulty settings and modifiers for better loot. There is a campaign with story missions that you can coop if you want but are primarily single player. The mission you play at the start of this beta is a story mission.
I have same question. Is there like a hub or something or just a bunch of instanced mission maps that I pick from menus?
There's a social hub called the farm with vendors and faction stuff, a "patrol" which is basically large open world maps that will have public events, bounties, small quests and stuff. There are also the story missions which you pick from a menu.