Select all, how to then deselect multiple (rows of) files?

Say I select all files, being displayed as small thumbnail images
Probably some 20-25 percent is should then be deselected - different subjects.

Whilst selecting one can either select all, or also entire rows, or multiple adjacent thumbnails using ctrl-shift.
Perfect.

However, how to deselect rows?
Right now, the only way I know is to click on each individual thumbnail in order to get them deselected.
Vainly tried to figure out how to deselect entire rows though, or multiple adjacent thumbnails.

Thanks!

Hold Ctrl and drag across a row. I think Ctrl+click and then Ctrl+Shift+click may also work.