I can import photos from another catalog into my main catalog using File > Import From Another Catalog. I can export the keywords from the other catalog using Metadata > Export Keywords and import the keywords into my main catalog using Metadata > Import Keywords. I am then left with the imported photos and the imported keywords but none of the keywords are assigned to any of the imported photos.
If anyone knows of a way to preserve the keyword assignments when importing from another catalog, I would be delighted to hear from you. Thanks.
By the way, the same problem applies to collections.

Rob_Cullen • Community Expert , Jul 07, 2023
"File > Import From Another Catalog" will bring with it keywords attached to photos, but, if you only want a selection of photos (and not every photo) copied to the secondary (existing) catalog [B], you would need to-
-first make the selection in catalog [A],
-and then "File > Export as Catalog" to a transfer Catalog [C],
-Now in catalog [B] "File > Import From Another Catalog" from the transfer [C] catalog.

"Export Keywords and ...... Metadata > Import Keywords." Does not keep keywords with files- true. Only the keywords= list is copied. Such as when creating a new empty Catalog, you might want all your keywords available in the new catalog.

You could also select photos and-
[Metadata > Save Metadata to file] in Catalog [A]
[Metadata > Read Metadata from File] in Catalog [B]-
"the same problem applies to collections."

Collections are merely lists of selected photos in the Library. And keywords assigned to a photo in a Library folder will also show when viewing the same photo in a Collection.

Just to add to @Rob_Cullen's correct explanation, importing catalogs works and does what you want, because all of your keywords and collection information and edits are in the catalog file. Importing photos does not work. (Importing a catalog is not the same as importing photos).

Yes. Export the folder as catalog and use 'Import from another catalog' in the other catalog. That will bring over the images and the keywords (and the edits).