You can add or remove buttons in the left side menu. Tap on the 3 lined button in the top left of the screen to open the left side menu. Then scroll down to the bottom and tap on "Edit settings". View
You can add folders, accounts, searches, folder filters, calendar, a browser and much more.
You can also create folder filters to include different accounts and folders and add them to the left side panel. (View)

The main popup panel menu is opened with the button in the bottom right of the screen with the four squares. Tap to see the available panels and choose one. You can change the panels menu with the 'Edit' button in the top right of the panels screen. You can also edit the panels in the left side menu - settings - 'Panels'. You use the same edit list screen as the side manu. (View)