Microsoft 365 Developer Platform could help you extend Microsoft 365, not necessarily Office apps, you could build solutions for Microsoft Teams, Microsoft Graph, SharePoint Framework, SharePoint Add-ins, and more.
More information, please refer to the article "Use your developer subscription to build Microsoft 365 solutions".
If you have other questions related to Microsoft 365 Developer Platform, please get support from link: Office developer community, as we are mainly responsible for general issues of Office desktop applications.
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