There are multiple ways to select objects in AutoCAD. Here are just a few:
Click objects individually. Once selected, objects should remain selected.
Click in the upper left corner to start a rectangle window and then click in the bottom right corner to create a selection window. Anything completely inside the window will be selected.
Click and hold to start the lasso then drag your cursor to create a free-hand selection set.
The Quick Select tool makes it easy to filter a selection by specifics such as object type and properties.
1) Activate the Quick Select by right clicking in the workspace and picking the Quick Select option OR by typing in "qselect" and hitting enter
2) Specify Object type OR specify Properties
3) Then click OK to make selection
Notice only objects in the drawing with those specifications will be selected. The total number selected will show above the command line.
Grip controls appear on objects as little blue squares once the object has been selected. They can be selected and then used as handles to modify the object. Or if the cursor is hovered over a grip, it will show measurements.
Collin, Dennis. Cadline Community. 6 Sept 2019. https://www.cadlinecommunity.co.uk/hc/en-us/articles/360002429817-AutoCAD-Getting-to-grips-with-Grips-