How do I restore scrambled Photos and recover missing iMovie after macOS Ventura update on iMac 5k Retina?

Upgrading your iMac to Ventura should not have deleted iMovie.

If your old copy of iMovie came from the App Store, you may be able to download a new copy of iMovie by going to the Purchased section of the App Store. If you have never downloaded iMovie before (e.g., if your copy came from DVD, back in the days when Apple distributed iLife that way), then you are probably out of luck, as the current App Store copy of iMovie requires macOS 14.6 (Sonoma) or later.

Were you backing up your computer before you did the Ventura upgrade?

What version of macOS were you running before?

The iMovie that came on the iLife DVD was 32-bit, and would be incompatible with macOS Ventura.

Ventura should not have deleted iMovie, but instead replaced its application icon with a translucent circle and slash visually indicating it was incompatible (and unusable). Since iMovie (e.g. iMovie '09) came from a DVD, there would be no record of its installation in the Mac App Store. If Ventura did not automatically install its own compatible version of iMovie, then you are out of luck as the current version in the Mac App Store requires macOS Sonoma v14.6 or later.

The older 32-bit iPhoto application was supeseded by the Photos application back at macOS 10.3. The Photos application in Ventura is an entirely different application to iPhoto, but can import that iPhoto library that should still exist in your Pictures folder. You do this by pressing the option key when launching Photos and choosing the iPhoto library from the following dialog:

What do you mean by "scrambled".

Please explain the problem.