OneNote 2021 v2103 is actually generated using OneNote v2103 in Microsoft 365 until 2021 march , repackaged, and changed the year number.
There are new features compared to OneNote 2016, as well as removals.
Overall, little has changed.

OneNote 2021 and OneNote 2016 in the Microsoft Support Documentation

 

Online Video

In “Insert” tab, add new group “Media” , add new “Online Video” feature.
This feature copies the video link from the specified video website and pastes it in to display the video directly on the OneNote page.
 
 

 

Check Accessibility

In “Review” tab, add new group “Accessibility”, add “Check Accessibility” feature.
 

 

Help Tab

Add new “Help” tab. It include follow features:
Help, Contact Support, Feedback, Show Training.
 

 

OneNote Feed

 OneNote 2021 Preview 2104 add Feed feature.
The Feed feature is used to view notes from Windows Sticky Notes, Outlook Notes, Samsung Notes, OneNote.

Feedback

Include: Send a Smile (I Like Something), Send a Flown (I Don’t Like Something), Send a Suggestion.
 

 

Update Options (Office Update)

Updates are automatically download and install.
 

 

New Features in OneNote Options Window

 

Add Black Theme, Remove Office Background

In General tab:

• Office theme, add “Black” theme.
• Delete Office background.

 

 
 

Sync Tab

Include 2 items:

• Sync notebooks automatically.
• Download all files and images.

 

 

Remove Help Language in Language Tab

 
Local help documents may no longer be available, and all help language settings are removed.
 

 

Pen Options

Add an option for pen in advance tab:

• Show Tablet PC Input Panel on OneNote pages.

 

 
 

OneNote 2021 is Missing from OneNote in Microsoft 365 Over the Same Period

Missing Search Box in OneNote Main Window Title Bar

 
This search box is used to search for features that come with OneNote.
OneNote 2021 does not have this search box.