Because there is a pressure difference between the two ends of the tube, a column of fluid can be maintained in the tube, with the height of the column proportional to the pressure difference. If the closed end is at zero pressure, then the height of the column is proportional to the pressure at the other end. In an open-tube manometer, one end of the tube is open to the atmosphere, and is thus at atmospheric pressure. The other end is connected to a region where the pressure is to be measured. Again, if there is a difference in pressure between the two ends of the tube, a column of fluid can be supported in the tube, with the height of the column being proportional to the pressure difference.
Many pressure gauges give only the gauge pressure.
The buoyant force applied by the fluid on the object is directed up. The force comes from the difference in pressure exerted on the top and bottom of an object. For a floating object, the top surface is at atmospheric pressure, while the bottom surface is at a higher pressure because it is in contact with the fluid at a particular depth in the fluid, and pressure increases with depth. For a completely-submerged object, the top surface is no longer at atmospheric pressure, but the bottom surface is still at a higher pressure because it's deeper in the fluid. A basketball floats in a bathtub of water.