Pages remembers the location where it saved last and that becomes the default for open events. Since the Finder Window sidebar Recents favorite is not a filesystem location, you cannot save a Pages document to it, nor can it be a configurable default open location. I never enable the Recents Favorite in the Finder since individual Finder windows can be organized by name and sorted by Date Last opened, or Date Last Added… among other settings.
The Pages application provides you a list of recently saved documents in its Open Recents… menu which depending on your System Settings > Menu Bar : Recent documents, applications, and servers setting can be up to 50 documents.
I would encourage you to keep your operating system up to date as there are considerable bug fixes, security, and performance enhancements between 15.6.1 and the current 15.7.