I know on iOS I can swipe down on the message I’m writing while writing an email (this hides the keyboard), then hit the calendar icon at the bottom of the screen. From the calendar view, if I hit the email icon I’m brought back to the draft I was just writing.

I have no idea if the same convention will work on Android, though.

If you add your fastmail account to iOS it’ll populate the inbuilt calendar and mail apps. Then you’ve got separate apps for each in addition to the fastmail app. Use whichever ones you please.