Each player account has a personal bunker, shared will all their characters. There are 7 types of rooms accessible with buttons at the top of the screen. Opening each room will enable you to use it's functionality.
Bunker rooms
World
Allows you to exit the bunker with a chosen character.
Quarters
Allows you to create and delete characters. You can have a maximum of 3 characters.
Storage
Allows you to store items. There are 7 pages of item slots that you can use, with each page having 12x11 slots, for a total of 924 storage slots. Right clicking a tab will enable you to rename the page.
Workshop
Allows you to craft items.
Science Lab
Allows you to research new technologies.
Medical
Currently it has no use.
Generator
Currently it has no use.