If you still have a copy of iPhoto.app in your Applications folder, you can trash it. The app is rather large, more than 1GB of storage, and you can free this storage.
The photos and videos stored by iPhoto are in the iPhoto Libraries, as Richard explained.
Even if you are having huge iPhoto Libraries, the storage for identical photos and videos is shared by iPhoto and Photos. If you already migrated the iPhoto Libraries to Photos and your photos are both in iPhoto and Photos, they are using the storage only once.
I recommend a test to check, which photos and videos from your iPhoto Libraries are missing in Photos. On macOS 13 you can simply launch Photos, then use the command "File > Import" and select an iPhoto Library for importing. Then wait. Photos will now show you all photos from the iPhoto Library, that are missing in your currently open Photos Library. To save them in your Photos Library, click "All new Photos" or select the photos you want to import.
Is the system version given below your question still valid? macOS 10.12? I would expect a new iMac to come with macOS 12 or 13. You need at least macOS 12 to import photos directly from another Photos Library or iPhoto Library.