Understanding photo libraries when migrating from iPhoto to Photo I recently updated an outdated OS to Catalina, in which the iPhoto app was replaced by the Photos app.
In HD/Users/User/Images, I now find three different folders :

iPhoto-Library (Migrated Photo Library) - 45GB

iPhoto-Library (Photos Library) - 40GB

Photos-Library (Photos Library) - 50GB - contains some 150 more photos than iPhoto-Library - Not sure if the Photos-Library was created automatically or by me.


I know I can open one of these libraries at a time in the Photos app. I also know that in the Finder, I can right-click and access the structure of these libraries. I noticed that they do not have the same folder structure.
My goal is to obtain one Photos-Library containing all the photos contained
in the three libraries (including a check so as not to lose possible non-duplicates).
Where can I get more information about how to proceed?
On this Stackexchange, I found several questions related to my issue, but I haven't found the exact solution. Some links :

How to duplicate-check between iPhoto Library and Photos Library *and* move non-duplicates from iPhoto -&gt; Photos?
If I&#39;ve migrated from iPhoto to Photos, can I safely delete the migrated photo library?

EDIT
I have added the file sizes in the list above.
The second linked question indeed answers my question more or less. It seems I can delete the iPhoto-Libraries and keep the Photos-Library.
I still do not understand the difference in file size, and would like to see which photos appear in the Photos-Library but not in the iPhoto-Library (maybe I have imported them before noticing there were different libraries). However, it seems there is no easy way to compare two libraries, as only one library can be opened at a time (except using external software, cf. the first linked question).
Some more information about the structure can be found here. The three folders you see all reference to the same set of image files on your Mac, using hard links.
The 150-photos difference MAY be due to one of these reasons:

Your Photos library already had some photos before you migrated iPhoto library (More likely).

You had RAW and JPEG pairs for some photos. iPhoto counted each pair as one photo, but in migration they exported separately, increasing the count (Less likely).


The Photos library should be the superset containing all your photos, but the only confirmatory test I can think of is to extract the file lists from the SQL databases of those three folders and compare them.