I have searched and not finding a thread that covers this, though I can't be the only one . . .
I have a DropBox folder created by the app on my Mac - I have been using this for years - files I add to this on my desktop iMac, or Mac Studio, were always readily available via the DropBo app on my iPhone, or iPad - to be used or shared. Something has changed recently, so that neither single files, nor folders of files added on the desktop are available within the DropBox app on the mobile devices. The only place these files seem to exist is the desktop Mac I created them on.