Scanning old photos directly into iPhoto using iPhone

iPhoto is an app that was discontinued 10 years ago, and it won't run on macOS15.1. Do you mean the app Photos?

If you're using iCloud Photos, then pictures you take on your iPhone will automatically appear in Photos on your Mac (after some delay, depending on the battery and internet situation.)

Taking pictures of old pictures can be a little tricky…

iCloud Photos is designed as a synchronization service. When you engage iCloud Photos on a device, then the Library on that device is kept exactly the same as the iCloud Photos Library. So, for instance, if you take a picture with your iPhone, it is added to the iPhone's Photos Library, copied to iCloud Photos Library, and then copied to the Photos Library on each of the other devices that you have connected. If you delete a picture on your Mac, then that picture is deleted at iCloud and on all the other devices.

Some people misunderstand iCloud. Keep in mind that iCloud Photos provides a sort of backup, but only partly. If your Mac gets run over by a truck, then your files will still be available through iCloud Drive. But if you accidentally delete all the pictures of your wedding, don't expect to go to iCloud and get them back-- they were erased there, too. So we use other backups for precious pictures.

On a Mac, you can have more than one Photos Library, but only one of them can be connected to iCloud. So, if you want, you could keep your usual pictures in a separate Library, and synchronize only your scanned pictures.