Saving is not done by the Go or the Mobile app but by the iOS; it's automatic after 10-12 seconds of idle time. If you’re not sure if you want to keep what you’ve just done, make a copy of the script in the Script List view.
FD Go and FD Mobile use the Files app in the iOS. More information about file management from Apple is here: https://support.apple.com/en-us/HT206481
5078
https://fd.support/3Nhwepl