If you need to select multiple objects, you have several options. Either hold down the Shift key and click on the desired objects. In case you click on already selected object, it will be removed from the selection. Or click to blank area (even outside the page) and drag – all the objects that will fit into the frame so created will be selected.
You can also combine these two approaches. If you already have multiple objects selected by dragging the mouse, you can add or remove object via clicking on them while keeping Shift key down. And vice versa: if you already have some objects selected, you can start dragging while Shift key is down and objects in created frame will be added to the selection.
You can also press Ctrl + A (on macOS: Cmd + A) to select all objects – either all on the page, or all in the open group.
Selected objects can be moved by mouse by dragging on any of them, eventually shifted via keys arrow up/down/left/right. There are coordinates of the upper left corner of the area that surrounds the selected objects shown in left sidebar. Changing these values you are able to move objects to exact place. As in case of all numeral inputs you can use keys arrow up/down for changing values, eventually enter a calculation (f.e. 100+50 is calculated as 150 automatically).
Selected object can be also grouped, see Objects groups.