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.
There is no conversion from kilograms to liters or vice versa. The kilogram is a unit of weight or mass, whereas the liter is a unit of volume or capacity. In order to determine the weight of a liter of something, you must know what that something is.