How can I migrate Google Fit activity data to Apple Health? Is there a first-party way of doing this?  I exported all my data from Google using the Google Takeout interface, but this produced a zip file, which iOS cannot open.  I also cannot install Google Fit on my iPhone to be able to import the data at the application level (it is not available in the Appstore).  

How do I get the data into Apple Health?
You can't.  Even with third party apps.