Metadata can be added to all the images in your catalog if you want.
Turn off filters, then click on All Photographs, then select them all (Ctrl-A on Windows, I don't know on Mac), then add the metadata. This will probably take a while.
Watermarks can only be applied when you export a photo.
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It is not recommended to export these many images and add watermark to them in one go.Please add watermarks to batch of your images,Preferably create collections and export them with the watermark added.
And if you consider watermarking in Photoshop be aware that Photoshop can only edit one image after the other – so while one can Batch or otherwise trigger the editing of multiple images they will still each have to be opened, edited, saved (or saved as/exported) by Photoshop.
Which naturally can take a while …