How do I favorite more than one photo at a time?

. Is there any way to favorite multiple photos at a time or do I have to continue to view them one at a time and mark it a favorite?

You can mark several selected photos at once as favorites by pressing the period key ".".

And while it's slightly off topic to my original question, any word on a way to permanently delete all the folders Apple thinks are useful, like screenshots and selfies?

The predefined albums are smart albums that will be hidden, if none of your photos is meeting the constraits. As long as you are having screenshots in your library the screenshot album will be visible, and as long as you are having photos taken with the front camera of a mobile device you will have a selfie album.

Apple is creating these additional albums because the smart albums the users can define are somewhat limited. When the "selfie" album was introduces, we had no possibility to create our own album to recognized front camera photos. Now it is possible by searching for photos taken with a certain lens. It is still not possible to define a smart album for screenshots, bec ase they do not have exif tags that are available in the smart album constraints. It would not be a problem, if the smart albums were more versatile and gave access to more EXIF and IPCT tags.

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You can mark several selected photos at once as favorites by pressing the period key ".".

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And how can I do that in an iPhone in the absence of a physical keyboard?!

that would be a question for the iPhone forum not for the Photos for Mac forum where you posted

LN