Which system version do you have installed on the old Macs on the Mac you are currently using? It will depend on the system versions you have installed on your Macs on how to get your iPhoto Libraries converted to Photos Libraries.
You wrote "I find a few old photos in iPhoto and Photo apps". Just to be sure - you need the iPhoto.photolibrary and the Photos.photoslibrary where the photos are stored to retrieve the photos.
- If the system version on the old Mac is MacOS 12 or older, you can open the iPhoto Library in Photos and create a new Photos Library from the iPhoto Library.
- If the system version on the old Mac is macOS 10.14 Mojave or older and iPhoto is still running, open the iPhoto Library also in iPhoto, so you can compare it to the new Photos Library and fix migration errors. Most of the smart albums will need fixing, because the rules are different in Photos and iPhotos. Also many of the named faces may be wrong or missing, and the descriptions of events and albums will need to be saved somewhere else. To be able to open the iPhoto Library again rename the filename extension from .migratedphotolibrary back to .photolibrary, or iPhoto will not be able to open the library.
- Give the new Photos Library a unique name, then transfer the new Photos Library to your new Mac.
- If the library has been created on macOS 10.14 or earlier, it may contain videos and image files in a legacy format, that can no longer be read on macOS 10.15 Catalina or later. You will have to convert such files to a supported format, before you try to use the library on a Mac with a newer system.
If you are having the full version of PowerPhotos, you can now follow the advice of Yer_Man and use it to merge the two libraries. It will depend on your system version on the new Mac, if PowerPhotos can directly open the iPhoto Library or if you first have to create a Photos Library from it.
Without PowerPhotos you can only import the new Photos Library into your current Photos Library, but that will import just the media (photos and videos) but you will lose the structure of the library (albums and folders) and the keywords.
If you are using iCloud Photos, you can merge the libraries by making the new library with your iPhoto Photos your iCloud Photos Library. iCloud will then upload the new library into iCloud and merge it with your existing iCloud Photos Library. This will save the structure of the library, also the keywords and named faces. The merged library will be downloaded into the currently open Photos Library.
