Deleted 2 posts because they resulted from me being confused about how these things work. I think I see how this works now. It does not seem to be possible to export a spatial bookmark series - created from features - as a bookmark file. Only bookmarks can be exported this way. I think that, in order to load bookmarks into a layout, I need to create them in the map first, otherwise the layout doesn't 'see' them as bookmarks it can load.
Just wondering if there is a way to load 1,500 features directly into a map without having to do it one at a time. I need to update a large number of features. I suppose I could load them into a layout, zoom to each, then activate the map for editing, return to layout, zoom to next feature, activate map again...
If I'm missing something here, that would not be the first time.