Starting with Symfonium 5.8.0, you can now select multiple items and perform actions on all of them.
This is available in most vertical lists and in those lists replace the long press drag and drop when enabled. (That feature is still available in a few other places).
You can start the multi select by either clicking the multi select button in those screens. (Click again to stop multi select)
You can also long press any item to start that mode and select that item.
Note: You can disable this in Settings/Interface settings/Media lists.
When in multi select mode, selected items have a distinctive background and a check mark.
You can click items to toggle their selection state.
You can use the bottom right action row to choose the action you want to perform on the items. Actions depends on the selected items and may change during selection.
Press the 
to stop selection mode and 
to unselect all selected items.
The feature support drag select to quickly select multiple items via gesture. Start long press on an item and move your finger while pressed to select more.
You can also start the gesture on an already selected item to start an unselect drag gesture.
Video: