It’s a design copied from a tutorial as their version is a lot more compact than mine. Basically though, when a hoppeinecart with items in goes over a regular hopper it drops 1 item in to it. This item is them put in to the furnace to smelt. Where this design gets clever is that because a dried kelp block can smelt 20 items it puts 20 kelp in each furnace every time it activates. At the basic level this is just maths - A hopper minecart hold 5 stacks (5x64 = 320 items) so run that over 16 furnaces and you get 20 items per furnace. The clever bit (and the bit smaller than my design) is it holds the hopperminecart until is full from the farm. It will loop round and round, depositing 1 item in to each furnace every time until it is empty. When empty it goes a different route to the filling station. It sits there until the farm fills is back up and then it will automatically be released to fill the furnaces again.
You could just collect the items straight away but I like the industrial piping look. So all the dried kelp goes in to a dropper which with a little redstone will output anything put in it. This will float up the water stream, across the water stream over the top of the kelp farm and be stored in the chests at the front of it.
If you want to see it in action, if you go down the hole at the front of the farm there are chests full of kelp from when I initially built the farm. Take some of it and place it directly in to the hopper minecart, as soon as its full it will set off. Just don’t stand on the rails to do this like I did....errr I mean some other idiot did
As for iron, I might have enough actually. I made enough hoppers for a fuel line too but didn’t build it. I’ll check later and let you know.