Elements will normally be positioned right where you drag them. Video's and images can also be set to fullscreen when they're not inside a group. That way they can cover the total background of your browser window. They have the same scaling settings as backgrounds.
You can select multiple objects by pressing the shift key while clicking them. You can then move these objects. You can group them by using the right mouse button and select 'group'. The group now gets an orange outline. To select an object inside the group you now have to double click to enter the group and then click the object you want to select. You can also use the layer panel to select an object inside a group.
Groups can have a fixed position (default) inside the safe zone or they can 'float' as we like to call it. It doesn’t have anything to do with the css property float but will let an elements position be calculated from a certain point like the right bottom corner or right middle. That way a group will stay at the right top of the screen even when the ratio and size of a screen will change. The settings are in the object panel in the second sub tab. When selecting float you can also set some margins for positioning.