What is this fire pipe valve? What do several water levels in the fire pool mean?

Question 1 "The diameter on the left side is small, and the diameter on the right side is large" is concentric diameter change, depending on the specifications of the diameter change. Question 2: "What are the two valves on the left of 300 in the lower right corner?" The picture is not comprehensive and it is difficult to judge. According to common sense, gate valves generally come out of the water tank; Question 3: When the overflow level reaches this position, the pool water begins to flow out. The high alarm level should be the upper limit of the level controller, and the water level of the highest level will start to control the water inflow. Fire water level is the minimum water consumption for fire fighting, and the fire pump needs to stop pumping at the lowest water level. Please forgive my inappropriate words.