The main screen is the hub for pretty much all the features of the game. 

Divided on screen sides those are : 

The lower screen is where Home, Partner, Backpack, Campaign, Challenge, Guild and Starship menus are. 

In the middle / map area is where you can find the Heantunting Firm, Tower of Babel, Genetically Modified, Arena, Universal Shop, War Factory, Happy Hotel and recruit menus. 

On the top you can find the daily login calendar, the gacha draws, the 7 day goals, and the bonus purchases.

You can also find the coins and gems quantities as well as the profile. 

Those menus will be explored more in depth in their individual FAQ's.