Where is the Save button or What is the Keyboard Shortcut of Save in OneNote?

Question:
Where is the save button, What is the keyboard shortcut of save in OneNote?

Answer:
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.