This shackle is huge. That’s what it’s called, a shackle. A U-shaped piece of steel with a bolt that screws through the open end. See how big it is? Big enough to dwarf the hands of the men working it.
Big enough to hoist tons of steel. The sling is being shackled to the steel tab that is welded to the enormous piece of a ship. To shackle is to attach unfailingly. The name of this tool comes from the verb — it is a shackle because it shackles one thing (a sling) to another thing (part of a ship) in order to move it.
See how simple it is? Just two pieces of steel. One bent back on itself, the other straight as an arrow. And threaded. Screws are maybe the third-simplest mechanical advantage, after levers and wheels. Basic stuff here, a U and a screw, but they will hold massive weight. They will not fail. They will be assembled easily and disassembled easily and reused simply, too, again and again until the ship is finished.
All the technology, all the advancements, all the fabrication and design rely on astoundingly simple tools. In this case a shackle. And hands. Hands are tools, too. The trades people are the ones who learn to use them.