Tap the account area at the top of the screen to switch accounts.
You can access various functions related to Mastodon posts. You can scroll left and right for a number of tools.
Tools can be edited from　Toolbar settings
Slide the feather button up to post. The sliding action helps prevent accidental submissions.
In addition to the X button in the upper right corner of the screen, you can also close the posting screen by swiping down on the screen.
The content you are editing is automatically saved as a draft, and when you reopen the posting screen with the same account, it is automatically restored from the draft.