The only output format permitted by the ArcMap bookmark manager (v10.7) is an "ArcGIS place file (*.dat)" which is not entirely human-readable when opened in a text editor.

What is the file structure of an ArcGIS Place File? suggests it may not be possible to access content directly and probably requires ArcObjects to extract bookmark info.

Is there any other way to transfer bookmarks from ArcMap to QGIS?