Why two Photos libraries?

Hi,

I have an older iMac running High Sierra. Years ago, I used to use iPhoto, but about 4 years ago, I moved to Photos. I had some problems when moving to Photos, so I went to the Apple store and got some help. That is way my Photos Library is titled "Photos Library (original 2) (rebuilt)" {see photo}. There are also some older iPhoto Libraries since we had so much trouble moving to Photos. However, we eventually got it working.

I do not use iPhoto anymore, but still have it on my Mac.

I did not delete the iPhoto library, as I got conflicting information about whether to delete it or not. So, in my Pictures folder there was an iPhoto Library (and other older iPhoto libraries) as well as a Photos Library (original 2) (rebuilt).

I am using Photos 3.0.

Recently, I noticed that there are now two (2) Photos Libraries--the one highlighted above, which says (System Photo Library), which has all of my photos (around 9000), and the one entitled Photos Library (orignal 2) (rebuilt). I am not sure how long these two Photos Libraries have been there.

If I hold down the Option key at Photos startup and select the System Photo Library, everything is there. If I hold down the Option key at Photos startup ans select the Photos Library (original 2) (rebuilt), I only see about 7000 photos, which go through July of 2015.

One last thing--when I am in the System Photo Library and delete or edit a photo, and then look in the Pictures folder in the Finder, the "Date Modified" does not change in the System Photo Library, but it does in the Photos Library (original 2) (rebuilt).

Questions:

1. Why do I have both of these Photos libraries?
2. Can one of them be deleted without loss of some of my photos?
3. Can I delete all of the old iPhoto Libraries?

Thanks so much! -Joe