Restoring photos from iPhoto mergedlibrary

Recently we had a major issue when upgrading to Big Sur - the iMac crashed, and required a disk wipe and clean install. Our backup files contain the iPhoto mergedlibrary, but how do we get Photos to open it? It has all the album names, etc., just none of the photos which are in this 215GB file. The last two years or so of photos were on the iCloud, and we have those. It is the trips and weddings and stuff on the old iPhoto file we need to get now.

We have a late 2015 retina 27" iMac running OS 11.3.1