Welcome @Khaled. If you have copies of all the photo libraries you should be able to merge them with software like PowerPhotos - Merge Mac Photos libraries, find duplicate photos, and more.
Q1. Your Mac has a program named Image Capture. It will allow you to download the photos from your IOS devices to a location of your choosing on your Mac using a lightning to USB cable.
Q2. Once you connect a drive containing the library/libraries from other computers, hold down the Option key while opening Photos. You will then be able to select the Photos.library you wish to open.
Q3. After you finish the merge if, for example, the consolidated library is on an external drive, then anyone with the same version of Photos.app should be able to access the photos. If this isn’t possible, one alternative would be to export the photos to a folder for them.
Here’s some threads and links you might find useful.
This is how I would approach the problem. “Stay tuned”, others on the forum may have other/better ways to accomplish the task.