Questions about Valves

The attached picture of a vertical Webstone valve was taken underneath our wall hung Triangle Tube Instinct Solo boiler. The valve is in the pipe that returns water to the heat exchanger. With the system full of water and the boiler turned off, if the valve's main handle is moved to the closed position and then the drain valve is uncapped and the drain valve handle is moved to the open position, where is any water that then runs from the open drain valve coming from? Is it coming down from the heat exchanger or up from the one-inch pipe that supplies the heat exchanger? I'm trying to understand the operation of this valve without having examined it prior to installation.

I have the same question about the horizontal Webstone valve in an attached picture. With the system full of water, if the valve's main handle is closed and the drain valve is uncapped and then opened, if water then runs out of the drain valve, is it coming from the circulator to the left or the supply pipe to the right?

When I understand the operation of these valves, I hope to be able to flush out the existing system water which needs to be replaced. Thanks for reading and any explanations.