Where is the save button, What is the keyboard shortcut of save in OneNote?
There is no Save button in OneNote.
But, OneNote provide Save Keyboard shortcut.
OneNote save keyboard shortcut is: Ctrl + S
While OneNote is running, your notes are automatically saved whenever you change them. Manually press Ctrl+S saving notes is not necessary.
Save Difference with Sync:
Save does not equal synchronization, if you want to sync after save note, press Ctrl+S, then press F9 to start to sync notes.