When you select an entity and click the little purple box icon next to the ones you've circled, it will drop to the ground at a height specified in the preferences (click the "0" button to make it drop all the way).

If the big down arrow (the left button you circled) is turned on, it will be randomly scaled and rotated.

If the little down arrow (the right button you circled) is turned on, it will align itself to the slope of the ground it lands on.

Instead of pressing the purple box icon, you can ctrl+alt+click on the ground (copies the currently selected entity and drops it where you clicked), or ctrl+alt+middle click on the ground (moves the currently selected entity to where you clicked).

[IW] Slime