Users have had the ability to edit MS Word, Excel and PowerPoint files directly in Google Drive for a number of years, Google making this the default behaviour in 2020. There aren’t currently any APIs or Apps Script services that enable you to edit these particular document types. This however hasn’t stopped Kanshi Tanaike for developing and sharing the MicrosoftDocsApp library which makes it possible to edit MS Word, Excel and PowerPoint files using the same methods as you would for Google Docs, Sheets and Slides.
The library is able to do this by converting the Microsoft files into the Google equivalent, before using the MS export options in Google Drive to replace the original files. This does create some limitations highlighted in the post. In particular, the original documents are overwritten and not directly edited which means any incompatible feature as part of the Microsoft to Google conversion will be lost. Regardless of these it’s a clever approach and might be useful for users who have to keep feet in both the Google and Microsoft worlds.