How to merge several libraries to one photo library?

Do you have albums and folders in your Libraries? If so, then I'm afraid that the method provided in Niel's link

Import photos from another library in Photos on Mac - Apple Support

will not preserve the structure of imported Libraries. If your albums and folders are important, then the best way to preserve that organization is to use the trusted 3rd party app PowerPhotos ($30) 

PowerPhotos is used by many of us who have multiple Libraries, and it does a great job of combining Libraries or transferring individual albums. It also offers a large number of other operations for Photos.

By the way, you can freely operate multiple Libraries on a Mac, switching between them at will, if that would be useful for you.

Click here and follow the instructions. If you have enough iCloud storage space, you can instead set each as the System Photo Library, upload its contents, and download the merged one.

(259801)

If you have a lot of organizational effort put into the various libraries you may want to use the paid version of PowerPhotos. The following is a comparison of Photos import and PowerPhotos merge capabilities:

I ran a number of tests with a 3119 photo, 72 video and 97 keyword Photos library with keywords, captions, keywords and locations.  I merged it into an empty library using Photos and PowerPhotos.  This is what I found was imported by each;

PowerPhotos:           Photos:

Albums***                                          -----------

Smart Albums (as regular albums)** -----------

Folders Containing Albums -----------

Captions                                             Captions

Titles                                                   Titles

Keywords* Keywords (some but not all: …

…see Note *)

Original images                                  Original images

Edited images                                     Edited images

---------------                                        Locations

Favorites                                             Favorites

Can detect and exclude                      Duplicates

duplicates upon import or not

Neither method could import/merge projects. 

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library. 

Notes:

*The Photos app only imported 84 keywords our of 97. PowerPhotos merged 147 keywords. Photos only imported keywords that were embedded in the original file and not those added by Photos. PowerPhotos imported all of them.

**The original library had 90 Smart Albums. All were merged by PowerPhotos but, as indicated above, the Smart albums were brought over as regular albums. No album of any kind were imported by Photos. Also only PowerPhotos imported folders.

The original library had 34 regular albums and 1 folder with some nested albums. PowerPhotos merger all of them successfully.

The original library had 72 videos. Photos imported only 57. PowerPhoto got all 72 in its merge.

This method would be much quicker than going thru iCloud.