Move an item to a collection
Wanting to move an item to a collection, you need to drag the item to the collection in the left collection tree. The same action does not exist in right click menu, only add to collection in there and it made an item exists in multiple collection. The latter action is as same as tags feature.
You can hold down a modifier key to move the item instead, but that's not the default. If you want to do that via menus, use the context-menu option and then use Remove Item from Collection or press Delete on the keyboard.