How to move photos from an iMac to a flash drive

On the menubar, Go to File, Export. In this link, you can find more info about the possibilities, e.g. about file type (quality) etc https://support.apple.com/lv-lv/guide/photos/pht6e157c5f/mac

open the finder window of the flash drive. then open your photos app. click once on one picture.

then go to AppleMenu>Photos>Edit>SelectAll.

the border of all the pictures will be surrounded in blue. then use one picture to drag and drop from the photos app onto the flash drive's finder window. the other images will drag over with the single picture.

sorry, i misinterpreted your question. to move only SOME of the pictures, click on one of them, then hold the shift key and click the rest of the ones you want. then drag and drop the group.