These changes are exclusive to documents within applications like Outlook, Word, Excel, PowerPoint, and others.
For users, the transition of the default font from Calibri to Aptos will be the most noticeable change.
Other changes include a new colour palette, styles, and updated default line weights. Further information on these changes can be found on the official Microsoft site.
Interesting fact: Calibri has been the default font for Office since the release of Office 2007.
Learn more about the history of Aptos - A change of typeface: Microsoft’s new default font has arrived | Microsoft Design.