This article explains how native Google Drive files are migrated.

During a migration from a Google Drive to another system (OneDrive or SharePoint Online), native Google Drive files are converted to Microsoft Office documents. The following table shows how they’re matched up:

Native Google Documents Microsoft Office Documents
Google Docs docx
GoogleSheets xlsx
Google Slides pptx
Google Forms Not migrated (ignored during migration)
Google Drawing png
Google My Maps Not migrated (ignored during migration)
Google Apps Scripts Not migrated (ignored during the migration)
During migration from one Google Drive to another, native Google Documents are migrated as native Google Documents.