There are several ways a user can submit the form, one them is submitting through the device menu after filling up the form. To use this feature you have to enable it in the data connector settings while building the form in the Creator.
HOW TO ENABLE SUBMITTING FROM DEVICE MENU
Start by adding a data destination in your form. In the destination setting window make sure to check the "Use this destination to submit from device menu". For SharePoint and Google Submit, you can find it on the same window under "Submit options".
Click "DONE", this will save the destination and close the window.
Make sure to click "SAVE & PUBLISH" before exiting the form creator.
Once the user is done filling out the form, he/she can click the icon on the top right icon on the screen and select the "SUBMIT" option from the dropdown.