In the documentation about the access to Google Fit it appears that the interface to Google Fit is handled in a generic way that is not necessarily coupled to Google Fit as a data supplier only: https://developers.google.com/fit/android/get-started
Is my assumption correct that therefore it does not necessarily lead to an error when Google Fit is not installed, because the Google Play services are an abstraction over the Fit API? What does happen if I access Google Fit without Google Fit being installed?
There is no requirement for the Google Fit app to be installed to use Google Fit.
The Fit app is basically just a client of Google Play Services.