Hi PatsyKB51,
welcome to this Apple (user-to-user) Community!
Re: "iMac Photos - after OS update to Ventura - no more collections?
I used Collections to gather together certain groups of photos.) What gives?"
Seems "Collections" has been replaced by "Memories" yet they are not the same thing.
To check on how Photos are organised in MacOS Ventura:
See Apple support's online article: Browse your photos in Photos on Mac - Apple Support
If you wish: You can edit your memories to make them even more personal.
Personalize your memories in Photos on Mac - Apple Support
Here's a link to the Photos User Guide for MacOS Ventura: (You can click on "Table of Contents" at the top of the "Welcome" page that opens from the link below, to view topics leading to detailed articles).:
Photos User Guide for Mac - Apple Support
All the best :-)