There is a small door in the top for adding oil to lubricate it. The door open looking down at the shaft. A planer in a lower section of the mill. In other words, a 2X4 will actually measure out at 2 by 4 inches dimension. Another view of the planer.
Power is transferred to it via a flat belt to the shaft on the lower left.
From there it goes to feed rollers that feed the lumber into the planer. Wide planer blades then shave off some of the wood and leave a finished surface. Look at planed finished lumber in the store. If you see small long raised sections lengthways in the wood, the planer blade was damaged. An example of an up and down sawmill.