That would be the "sheet.appendRow()" in the google apps script !
You have several choices, but it does sound like you want a full database on your google sheet (CRUD - Create, Read, Update, Delete)
If you want to keep things simple, is it possible to store the "half completed" entry on the user's phone, using tinydb, then upload when all the information is available?
If not have a look here:
or search forum / google for other methods