You can google for the details, but this is I hope a simplified...

You can google for the details, but this is I hope a simplified explanation.

"Having difficulty getting my head around Green Hydrogen as an energy source."
Lets says you have a tank of Hydrogen.
When that Hydrogen is slowly combined with Oxygen, chemical energy and water vapour is released. The chemical energy is converted to electrical energy which can be used to heat things, or power devices or electric motors etc. The concept has been used in Hyrogen fuel cells for decades in the space missions.
The Green in the Green Hydrogen name is merely informing you that the Hydrogen was obtained without generating green house gases.

"Also having difficulty with rationalising a major Iron Ore producer becoming a major Green Hydrogen producer. "

At the moment, FMG digs up Iron Ore and puts it on someelses boat to send it halfway around the planet to be processed by someone else. This is required because the Black Coal needed to currently process the Iron Ore is not in the same location as the Iron Ore is.
The processing that occurs removes the oxygen from the Iron ore to create Pig Iron, which then can be processed into Steel.
If FMG also produces Hydrogen, then uses that Hydrogen (instead of Black Coal) to process the Iron Ore into Pig Iron and perhaps Steel, then FMG moves itself up the value chain, sell the steel for the same price as it currently is, while at the same time converting all the activity of transporting iron ore into FMG profit.