Hi Eve,
From the description provided, as you required to insert the sub bullets in the OneNote Notebook, we suggest you try the below steps to achieve your requirement:
Open the OneNote application and go to the page where you want to input the bullets> In the Home tab, click on the Bullets> To insert the sub bullets, click on Increase Indent Position to get the sub bullets.
For the shortcut key to achieve your requirement i.e. to insert the sub bullets, click tab.
Appreciate your understanding.
Regards,
Chitrahaas