If you hover over a part, you will see 1 or more dots. The one with the gizmo centered on it is the active snap point. If you want to switch snap points, click on a different dot. Once you've selected your snap point, click on the Snap tool (Magnet icon, second from the top on the left hand sidebar) or use the keyboard shortcut S. Now click the snap point on another brick that you want to snap your first one to. Generally there is only one point for the top or bottom of a brick. But with your grid set to medium (the default setting) your movements will be 1/2 a stud, so you can be fairly precise by snapping and then moving a brick.