Bulk merge items sub-folders into new folder
Assuming the structure is like

main-folder
    subfolder-1
        item-1
        item-2
    subfolder-2
        item-3
        item-4


and all the items are uniquely named, you can use

cd main-folder
mkdir ../target-folder
mv */* ../target-folder/


to move everything into a target folder on the same directory level as the main folder. If you just want to move everything into the main folder, just use

cd main-folder
mv */* .