Iron comes from Iron Ore via the Forge. Iron Ore can be found in Iron Nodes in the Stalactite layer and Lava layer. The ore has to be harvested with an Extractor, as the node itself cannot be pulled with Power Cells, it also cannot be placed. All Ores can be smelted into bars in a Forge for further processing.
It depends on the substance (water for example has 1000 grams to a liter, other substances have more grams or less grams to a liter depending on a property (called the density measured in grams / liter) of the substance.
The prefix in front of the unit tells you how to move the decimal. 1 milliliter = 0.001 liters because milli means thousandth. 1 thousandth of a liter = 0.001 liters. 0.1 liters is 100 times as much as 0.001 liters. Therefore 0.1 liters = 100 ml.