I don't have my old Mac up and running at the moment – but you probably had iPhoto set to import photos into its library.
If this library was anything like the default Photos Library.photoslibrary which Photos uses, it probably was set up as a package. A macOS package is entire subdirectory tree that the Finder disguises as a single file, to discourage you from messing around inside, and possibly breaking some relationship between the contents that the Mac relies upon. Macs often use packages to hold
- Applications
- Media (music, movie, TV show) libraries
- Photo libraries
My guess is that somewhere on your Mac – probably in your Pictures folder – you will find a file whose name starts with iPhoto, and which is a package. You can open up a package by selecting it, right-clicking, and selecting Show Package Contents, after which you can navigate inside and do whatever. Presumably the iPhoto library will contain copies of your pictures in standard formats (such as .JPG), although you may find that the way the files are named, and organized, might not be set up to be friendly for human consumption.
If you do find an iPhotos library package, I would strongly suggest not opening it up – but instead, copying it to an external drive (or another folder) and doing a Show Package Contents on the copy. That way, if you mess up the copy while you are looking for things, or copying or moving stuff out of it, you've still got the original iPhoto library from which you can make a new working copy. If you tamper with the original, and mess things up, and it happens to be your only copy, you're up the creek without a paddle.
I don't know if there is any automated way to convert an iPhoto library to a Photos library in Ventura, given that the migration to Photos happened several versions of macOS earlier. Maybe someone else will be able to comment on that.