What it says

The best collab, Va-11 Hall-A. Here we get a good taste of how progression in events is like in older servers. The map selection is laid out in a tree-like pattern where you need to use a currency to unlock the next map. 

Thankfully, this currency is gotten by mixing drinks in the cafe for clients. So after each mission on normal, you'll go back to the cafe to get to the more va11halla side of the collab. You'll have to listen to the stories of some of the characters featured, then serve them a drink that they request. After the story section, you'll be able to unlock the next map in the event.