How to consolidate multiple Photos and iPhoto libraries into one Photos library in OS Big Sur

The best way if you don't want to use iCloud temporarily is to do the following;

1 - convert all iPhoto libraries to Photos libraries.

2 - user the paid version of PowerPhotos to merger the various Photos libraries into one comprehensive library. This PowerPhotos help site describes how: Merging libraries. You can put the library wherever you want. Do all of the merging on the EHD and make sure it's formatted as shown in this screenshot:

and connected by cable directly to your Mac.

PowerPhotos will not include duplicate photos in the merge.

The procedure from Apple's support document (Combine libraries in Photos - Apple Support) will currently not work on macOS 11 Big Sur, if you want to export edited versions as JPEGs. Because of a bug in Photos 6 the metadata will be missing from the exported JPEGs. The merging with the full, paid version of PowerPhotos, as described by OT, will give a very good result, unless your main library is syncing with iCloud Photos and "Optimise Mac Storage" is enabled.

If you are using iCloud Photos, I would merge the libraries directly in iCloud, as described here: Notes on Merging Photos Libraries, 2019 V… - Apple Community

If you hold down the option key while starting photos you get the choice to open other libraries - either from the list provided, or by clicking "other library" if it is not listed. If you open an iPhoto library, photos will convert it to a format it can open.

It is a good idea to make sure you have backups of the iPhoto libraries first, as this process can (rarely) go wrong.

Or to convert an iPhoto library to a Photos library just drag the iPhoto library onto the Photos icon in the Dock (or in the Applications folder). It will convert it to Photos and you can proceed.